feat: expand Quick Actions with markdown, doc mutations, and undo

[SebTardif]

Summary

Adds 7 new Quick Actions to close the gap between the Patchloom CLI and the VS Code extension. These cover the CLI's unique capabilities that have no VS Code native equivalent.

Markdown operations (Closes #114)

• Append table row - md table-append: heading-aware table row insertion
• Upsert bullet - md upsert-bullet: idempotent bullet addition under a heading
• Replace section - md replace-section: heading-aware section replacement

Doc mutation operations (Closes #115)

• Delete structured value - doc delete: remove keys from JSON/YAML/TOML
• Merge into structured file - doc merge: merge partial objects into config files
• Append to array - doc append: append values to arrays in structured files

Undo (Closes #116)

• Undo last change - undo --apply: restore files from patchloom backup with confirmation

All operations use the existing preview-and-apply pattern with diff view (except undo, which uses a confirmation dialog).

Testing

• 10 new unit tests for builder functions
• 6 new CLI integration tests (doc delete, doc merge, doc append, md upsert-bullet, md table-append, undo round-trip)
• Total: 242 tests, all passing
• Coverage: above 80% threshold
• npm run check passes (compile + tests + coverage + package)

Quick Action picker now offers

#	Action	CLI command
1	Replace text in file	replace
2	Tidy file	tidy fix
3	Set structured value	doc set
4	Search text across files	search
5	Create a new file	create
6	Read structured value	doc get
7	Delete structured value	doc delete
8	Merge into structured file	doc merge
9	Append to array	doc append
10	Append table row	md table-append
11	Upsert bullet	md upsert-bullet
12	Replace markdown section	md replace-section
13	Undo last change	undo