docs(claude): add shared-query and persisted-data change rules

[giladresisi]

What kind of change does this PR introduce?

Docs — three new rules in CLAUDE.md.

Why was this change needed?

During the work on #1868, adding error: true to PostsRepository.getPosts was analyzed only through its calendar consumer — missing that the same query also serves GET /public/v1/posts, so the public API would have started returning the raw stored error column verbatim (including legacy rows holding full serialized workflow failures with stack traces and request payloads). The same PR also changed what Post.error persists without elevating that to an explicit decision.

The new rules make both mistakes mechanically checkable before they reach review: enumerate every consumer before widening a shared select/return shape, treat exposing a stored field to a new external surface as a data-exposure decision (including checking what legacy production rows contain), and never change persisted-column semantics silently.

Other information:

The #1868 leak was caught in review before merge; these rules are prevention for next time.

Checklist:

• I have read the CONTRIBUTING guide.
• I have signed the Contributor License Agreement (CLA).
• I confirm I have not used AI to submit this PR or generate code for it.
• I checked that there were no similar issues or PRs already open for this.
• This PR fixes just ONE issue

🤖 Generated with Claude Code

https://claude.ai/code/session_0181SwfBoof22oiPhfveYFej

[strix-security]

Strix Security Review

No security issues found.

_{Updated for 8025163.}

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