#587 Correct five comments that drifted from the code

Summary

Five docs-only corrections where a comment contradicted the implementation. No behavior changes.

1. crates/relayburn-sdk/src/reader/inference.rs — the request-id docs referenced the removed InferenceFallback enum and a non-existent row-uuid wire label, and claimed Codex/opencode turn_id equals request_id. The code uses InferenceKeySource, serializes row-synthetic, always sets turn_id from message_id, and synthesizes Codex request ids as {message_id}:request:{n}.
2. crates/relayburn-sdk/src/reader/types.rs — ActivityCategory::Skill was documented as a parentUuid structural triad detected by classifier::detect_slash_triads, which does not exist. Detection now comes from relayhistory slash_command markers via skill_messages.
3. crates/relayburn-sdk/src/ledger/db.rs — Connections::open was documented as requiring existing parent directories, but it calls create_dir_all for each path's parent.
4. crates/relayburn-sdk/src/ingest/pending_stamps.rs — resolve_pending_stamps_for_session_in was documented as applying every matching manifest. The loop breaks after the first successful append, so at most one is applied per call.
5. crates/relayburn-cli/src/cli.rs — the Command enum was documented as stubs that print "not yet implemented" and exit 1. Every variant is fully typed and dispatched to an implemented presenter.

Verification

• cargo fmt --all --check
• git diff --check

cargo check -p relayburn-sdk -p relayburn-cli --all-targets could not run on this Windows host because ring needs clang, which is not installed; it fails in ring's build script before compiling workspace code.

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