yield: timestamp-window commit attribution can cross-credit parallel sessions and states results as fact

[avs-io]

Problem

A commit is attributed to a session if its timestamp falls inside [firstTimestamp, lastTimestamp + 1h] (src/yield.ts:154). Attribution is per repo, falling back to cwd when projectPath is missing. Two sessions overlapping in the same repo each see the other's commits inside their window, so a session that shipped nothing can land in the productive bucket. The rendered output then states the classification as fact ("sessions shipped to main", src/yield.ts:236-238) with no caveat; the only hedge is the word (experimental) in --help, which never appears in the report itself.

Evidence

• src/yield.ts:152-160 (window), :199-205 (repo/cwd fallback), :236-238 (unhedged wording)
• Verified on main at 1a75484.

Proposed fix

1. Put a one-line attribution caveat in the text report and a field in the JSON.
2. Mark sessions with overlapping windows in the same repo as ambiguous instead of double-crediting.
3. Optionally tighten correlation (branch match, or commit author-time strictly inside the session span) before the +1h grace applies.