Look and Exec are builtins like any other

Their only special feature is now the keys they are assigned. `Look main.zig`
typed in a tag is the same look a right click is; config.look_cmd/exec_cmd
point the two buttons and Enter/Tab at them, so retargeting Enter to Grep is
one line. actOnSelection — a hand-written cascade with the builtin dispatch
nested inside it — is gone, and the `button` parameter it threaded through five
KEYBOARD call sites went with it.

New syntax, spelled once in config.zig: @`ls -la` names a command to run
rather than a file to open. Word expansion takes the quoted run whole, the way
acme does for its own </|/> words, so a click inside one does not hand Look the
fragment `ls`.

And it nests: @`Look .` unwraps, re-enters the dispatcher, and looks at the
directory. Guarded at depth 8, which is unreachable today (every re-entry
strips a word or a delimiter pair, so the string strictly shrinks) and exists
for a future syntax that does not shrink.

leader_path is now optional: a builtin may have no SPC path when its shortcut
is a key and a mouse button. New golden cmdword; the other 59 unmoved.

1 files changed, 217 insertions, 0 deletions

diff --git a/test/snapshots/cmdword.golden b/test/snapshots/cmdword.golden
new file mode 100644
index 00000000..03e272be
--- /dev/null
+++ b/test/snapshots/cmdword.golden
@@ -0,0 +1,217 @@
+== snap opened grid=120x30 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+== snap atexec grid=120x30 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+== snap atlook grid=120x30 cursor=64,8
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+|                                                              ZZL
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+== snap lookword grid=120x30 cursor=7,17
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZE
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+|                                                              ZZL
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap nested grid=120x30 cursor=64,10
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZE
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+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap execword grid=120x30 cursor=102,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          INS /tmp/pardes-snap/cmdword/cwd Del Exec
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZE
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+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap tagcmd grid=120x30 cursor=116,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          INS /tmp/pardes-snap/cmdword/cwd Del Exec @`echo ZZ''T`
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZE
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+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZT
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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