docs(product): align durable Session scope and acceptance

[Yevanchen]

Summary

Require both single-turn ghFind evaluation and CSV analysis with multi-turn follow-up through one durable Session API. A user returning seconds or days later must continue from the prior conversation, working files, and admitted configuration, including after runtime reclamation. Retain the checkpoint gate for truthful success and safe reclamation, with renewable recovery for at least 30 days.

Keep Agent invocation at the latest saved configuration with internal Session snapshots, full access within isolation boundaries, and immediately callable managed Agents. Record #581 as shipped, align the README/architecture/PRD anchors, and document Cloudflare notification requirements for future breaking releases. The #546/#582–#584 issue descriptions were reconciled with this reviewed scope on September 10.

Acceptance clarity

Actual continuation establishes acceptance; backup creation alone is insufficient. Verify modifications using prior context and working files outside published artifacts without re-supplying them. Separate controlled-clock retention tests from actual elapsed-time evidence for delayed live continuation. ghFind may complete its business task in one turn while its Session retains the same continuation guarantee.

Verification

• All seven original changed Markdown files passed focused formatting checks; the four September 10 follow-up files passed again after editing.
• just docs-check passed for 56 Markdown/MDX files; git diff --check passed.
• The previous PR head passed the Repository check and metadata/CodeQL gates in CI. Checks for the new head run on this update.
• The earlier local just check failed in unchanged Driver process-supervision tests on macOS (1,226 passed, 61 skipped, 11 failed, one unhandled error); the full local gate was not repeated for this documentation-only follow-up.
• The commit-msg filename hook again incorrectly rejected the worktree COMMIT_EDITMSG path as an illegal Windows filename. Only check-illegal-windows-names was skipped for the retry; other hooks and commit metadata validation passed.
• Issue bodies were read back and matched the prepared scope; all four implementation issues remain open.

Impact and review

This PR changes target documentation only. No runtime behavior, generated contracts, database schema/migrations, lockfile, environment configuration, production deployment, or email send changes. The local CLI provider smoke is not evidence of delayed continuation, and this PR does not claim implementation acceptance.

Default models/provider supply, budget values, and the smallest fixed-commit ghFind material contract still need resolution before their corresponding capabilities ship. Typed Git infrastructure, interactive approvals, and public historical-version invocation remain outside v1. Rollback is a documentation revert.

Refs #546, #581, #582, #583, #584.