feat: manage CLAUDE.md/AGENTS.md as a delimited begin/end block instead of overwriting the whole file

[dehuaichendragonplus]

Why this change is needed

Today SyncClaude / SyncCodex regenerate the entire CLAUDE.md / AGENTS.md on every sync (File.WriteAllText(path, BuildXxxContent(...))). Any content a user adds to these files — project architecture notes, coding conventions, pitfalls, other (non-Funplay) agent instructions — is silently destroyed the next time Project Skills syncs (after a package upgrade, or after toggling platforms).

Real incident that motivated this: in a production project, a hand-authored CLAUDE.md (hundreds of lines of project-specific guidance) still carried the Funplay managed marker at its top from an earlier generation. A later sync overwrote the whole file back down to Funplay's ~40 generated lines, losing everything. The single-line marker is an unreliable ownership signal — it survives a hand-rewrite of the body, so IsManagedFile reports "managed" while the body is actually the user's.

Root problem: Funplay treats these shared root files as wholly-owned, but they are shared surfaces users legitimately edit.

Change approach

Manage only a delimited block, never the whole file:

• Reuse the existing ManagedMarker as the begin marker (so IsManagedFile and version-status detection keep working) and add an end marker <!-- /Funplay Unity MCP managed project skills -->.
• WriteManagedBlock(path, title, block):
  • absent/empty file → create title + block
  • complete begin+end block present → replace only the block region; preserve everything before begin and after end
  • begin marker but no end (legacy whole-file marker, or an edited/corrupted block) → skip with a warning; its managed extent is unknowable, so refuse to guess and clobber
  • no begin marker → append the block below the user's existing content (touching nothing above)
• RemoveManagedBlock (platform disabled) strips just the block; deletes the file only if nothing but a bare title/whitespace remains. Files without a block fall back to the legacy delete guard.
• BuildClaude/CodexAgentsContent split into Build*ManagedBlock (block body only, no # CLAUDE.md title).
• Deliberately unchanged: IsManagedFile, the version-status / Upgrade Skills detection, and cursor rules. Status detection matches markers by content (Contains/IndexOf), independent of position, so it works whether markers sit at the top of a fully-managed file or inside a block.

Benefits

• User content in CLAUDE.md / AGENTS.md is never destroyed again — the MCP HTTP transport can fail to restart after Unity domain reload because previous listener is not fully stopped #1 safety issue with the current behavior.
• Funplay guidance stays updatable across versions (the block is replaced in place), so Upgrade Skills still delivers new guidance without a full-file rewrite.
• Disabling a platform cleanly removes just Funplay's block instead of deleting a file the user may have added their own content to.
• Backward compatible: a file Funplay fully generated in the old scheme (marker at top, no end marker) is treated as legacy and skipped-with-warning rather than mis-spliced; new syncs create the delimited-block form going forward.

Test results

Verified in a Unity 6000.3.13f1 project (package compiles with zero errors), then exercised WriteManagedBlock / RemoveManagedBlock via reflection over temp files:

Scenario	Result
absent file	creates # CLAUDE.md + block ✔
hand-authored file (no marker)	block appended, user content preserved ✔
file with block, re-sync	block replaced in place (v1→v2), user content preserved, old block gone ✔
legacy begin-only marker (no end)	unchanged / skipped with warning ✔
RemoveManagedBlock on block + user content	block stripped, user content kept ✔
RemoveManagedBlock on block-only file	file deleted ✔

Compatibility testing

• Only affects the claude and codex guidance-file platforms. cursor rules (Funplay-namespaced files under .cursor/rules/) are unchanged; LM Studio / VS Code / Trae / Kiro don't generate a guidance file at all (SupportedPlatforms = { codex, claude, cursor }), so they're unaffected.
• MCP client connection-config generation (FunplayMCPClientConfigPanel) is untouched.
• Version-status / Upgrade Skills behavior is unchanged and verified (detection is position-independent, so markers inside a block are detected identically to markers at the top of a fully-managed file).

Checklist

• Tested in a clean Unity 2022.3+ project (Unity 6000.3.13f1)
• Verified Funplay > MCP Server opens and starts correctly
• Did not commit local junk such as .idea/ or .DS_Store
• Updated CHANGELOG.md