refactor: pass envOverrides via tmux export instead of disk write

[TeigenZhang]

Summary

Before this change, POST /api/sessions persisted envOverrides (e.g. CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS) to <case>/.claude/settings.local.json so Claude CLI would pick them up on spawn. That had two problems: UI dropdown state could drift from the file on disk, and the backend was silently writing into user project directories as a side-effect of opening a session.

This refactor makes envOverrides ephemeral spawn-time state:

UI dropdown → POST /api/sessions { envOverrides }
           → new Session({ envOverrides })
           → session._envOverrides
           → tmux-manager.buildEnvExports → `export KEY=<shellescape(VALUE)>` prepended to the pane's spawn command

• Survives respawnPane cycles via session._envOverrides
• Survives server restart via SessionState.envOverrides in state.json
• Introduces session-ui.js#buildEnvOverrides(caseSettings, globalSettings) as the single source of truth for the payload
• Removes updateCaseEnvVars import from session-routes.ts (now unused)

Benefits

• No more stale .claude/settings.local.json files in user project directories
• UI state and spawn env are guaranteed consistent (one code path, one source)
• Frontend preview ("this is what will be exported") is trivially composable

Test plan

• Session spawn with agentTeams enabled → tmux pane sees CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 in env
• Respawn pane preserves env overrides across restart
• ~/.codeman/state.json contains envOverrides for each session, restored on server restart
• No writes to <case>/.claude/settings.local.json triggered by session creation
• npm run typecheck / npm run lint / npm run build pass

[Ark0N]

Thanks for this — the refactor is well-thought-out and addresses a real wart (silent writes into user case dirs). The tmux setenv approach is the right call: keeps secret values off the bash command line (ps-visible) while still surviving respawn-pane. The __envOverrides internal field with CLAUDE_CODE_*-only persistence filter is a sensible compromise between recovery and not leaking OPENCODE_* secrets into state.json. And the buildEnvOverrides helper is a nice cleanup of the four duplicated frontend call sites.

Two things I had to handle before merging:

1. Merge conflict with feat: thinking effort setting for new sessions (with xhigh/max) #73 (thinkingEffort) in src/web/public/session-ui.js. Resolved by keeping your buildEnvOverrides() helper — it already covers both CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS and CLAUDE_CODE_EFFORT_LEVEL, so the master-side inline block is fully replaced. Clean replacement, no behavioral drift.

2. Test fix in test/session-manager.test.ts: the MockSession class doesn't extend the real Session, so it didn't have getEnvOverridesForPersist. After updateSessionState started calling it, 19 tests failed with TypeError. Added a stub that returns undefined (matches the real method's contract for sessions without overrides).

Verified locally: typecheck, lint, format:check, build, and the four most-relevant test files (session-manager, session-state, tmux-manager, tmux-restart-recovery) all pass.

Merging now. 🙏