Strengthen nullable type discipline with true-myth and safety tests

[jhodapp]

Description

Establishes stricter nullable type standards using true-myth (Maybe<T>, Result<T, E>) and begins fixing the most dangerous || "" ID fallback patterns. Adds 35 safety tests that lock down null/falsy-ID behavior as a prerequisite for the broader migration.

Changes

• Install true-myth v9.3.1 for Maybe<T> and Result<T, E> types
• Update .claude/coding-standards.md with nullable type hierarchy: Maybe<T> → Result<T, E> → discriminated unions
• Update .claude/CLAUDE.md with top-level nullable type rule
• Replace 4 dangerous || "" ID fallbacks with render guards in coaching-tabs-container.tsx and page.tsx
• Remove 1 unnecessary || "" in coaching-session-list.tsx (hook already accepts null)
• Add 14 SWR hook null-path tests verifying no-fetch behavior with falsy IDs
• Add 1 test for CoachingSessionApi.list(null) early return
• Add 11 tests for sanitizeCallbackUrl null/undefined/empty handling
• Add 6 tests for buildInitialOrder null-return contract

Testing Strategy

• npm run test — all 612 tests pass (35 new)
• npm run build — compiles cleanly with no TypeScript errors
• Manual: navigate to a coaching session, verify Notes/Agreements/Actions tabs still render and function correctly
• Manual: verify agreements tab only renders content once the session ID loads (render guard)

Concerns

• This is the first phase of a larger nullable type migration. The assessment report (nullable-type-assessment.md, not committed) identifies ~65 domain-level violations across 6 tiers. This PR covers the standards + Tier 1 fixes only.
• The AgreementsList render guard changes the always-mounted CSS display pattern slightly — the component now mounts when the session ID becomes available rather than on initial render. Tab switch state preservation is unaffected since the session ID doesn't change during tab switches.