feat(session-context): resolve the active path per AI_AGENT_ID

A single .ai/session-context.org races when two agents share a project: each agent's writes clobber the other's session log. I added .ai/scripts/session-context-path, which resolves the active path from AI_AGENT_ID: unset gives the legacy .ai/session-context.org singleton (so every existing one-agent session is unchanged), set gives .ai/session-context.d/<id>.org with the id sanitized to filename-safe characters. This is Codex's Phase 1 slice from the runtime-neutral spec: the race fix on its own, no broader refactor.

startup.org's existence check and wrap-it-up.org's rename now resolve through the helper, each with a singleton fallback so older checkouts that haven't synced the script still work. Wrap folds the agent id into the archive name so two agents wrapping in the same minute don't collide. protocols.org documents the rule. Verified with 5 bats cases and a two-agent simulation showing distinct paths per id.

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