fix(config): stop disconnect destroying files the user wrote

[fstubner]

Two ways disconnect destroyed user content. Both found by driving a real setup/disconnect round trip, not by reading the code — the splicing helpers were individually defensible and the round trip still ate the file.

1. A file that was only the author's own frontmatter was deleted

Removal asked "is what survives any YAML frontmatter?" and deleted the file when it was. That cannot tell xtctx's own Cursor-rule prelude from a metadata stub someone wrote themselves.

CLAUDE.md before:  ---\ntitle: My own notes\ncustom_field: true\n---\n
after disconnect:  (file does not exist)

Now it compares against the exact prelude the writer prepends for that path — the only string removal is entitled to take back. The writer and remover already promise in their own comments to agree byte-for-byte; this is what that promise required.

2. An orphaned begin marker ate everything to the next block's end

The pattern was begin[\s\S]*?end, which pairs an opening marker with the nearest following close whatever lies between. An end marker lost to a hand-edit, or a merge that kept half of one side, made removal swallow the user's content.

before:  TOP LINE THE USER WROTE
         <!-- xtctx:begin -->
         stale text whose end marker was deleted

         ## The user's own heading
         Content the user cares about.

after:   TOP LINE THE USER WROTE

A begin that meets another begin before it meets an end is no longer treated as an opening. It stays as inert text — which inspectManagedFile already surfaces, and inert text is recoverable where deleted content is not.

countManagedBlocks now counts by the same rule, so status can't report a block removal refuses to touch and send the user to a repair that can never change anything.

Verification

Both cases now round-trip byte-identical through a real setup/disconnect. Tests assert at that level, not at the helper level.

mutation	result
restore the old regex pairing	2 tests fail
restore the loose frontmatter test	1 test fails

714 tests pass, typecheck and lint clean.

Deliberately unchanged

A file holding nothing but xtctx's own block is still removed rather than left as a stub. The --all global blast radius, duplicate-block self-healing, and comment preservation all behave as before — each verified still passing.