2026.08.16 - Implement Residual DCP Metadata Stripping

[lanhhoang]

Implement the plan in #53.

• Add DCP_RESIDUAL_INLINE regex (src/messages/strip.ts) to strip prefix-less -dcp-message-id> and dcp-system-reminder> residual fragments. Documented false positive: dcp-message-id foo>bar → bar.
• Extend stripHallucinationsFromString signature with optional knownRefs?: ReadonlySet<string>; add private stripKnownRefsFromString helper that strips bare m#### refs from the injected session set, sorted longest-first with word-boundary anchors.
• Wire the known-refs snapshot into the message_end handler in src/index.ts: notify "info" when the sanitizer changes the message, notify "warning" (defense-in-depth) when the sanitizer is a no-op but a residual pattern remains in the visible text.
• Add 8 + 8 + 4 tests covering the residual regex, the known-refs helper, and the notify branches. Existing 9 strip tests still pass.
• Add [Unreleased] changelog entry.

End-of-line residual regex (a separate DCP_RESIDUAL_EOL) was deliberately dropped per docs/07-addendum-residual-regex.md: a general EOL regex cannot distinguish truncated residuals from prose that mentions the namespace phrase. That case is covered by the looksLikeUnproductiveTurn-style warning notify instead.