Add CLI availability checks and provider opt-in settings

[mzhaom]

Summary

• Detect installed CLI tools (claude, codex, gemini) at startup via exec.LookPath and version probing; surface availability on the welcome screen and in repo settings
• Add EnabledProviders setting so users can toggle which providers appear in Alt+M model cycling — only installed+enabled providers are offered
• Move task router from wt/taskrouter/ to bramble/taskrouter/ and refactor it to use the generic agent.Provider interface, allowing any backend (not just Codex) to power routing decisions
• Extract AgentModel definitions and model registry into multiagent/agent/ for shared use across packages

Test plan

• scripts/lint.sh passes
• bazel test //bramble/... //multiagent/... --test_timeout=60 — 13/13 tests pass
• bazel test //wt/... --test_timeout=60 — 2/2 tests pass
• Manual: bramble → Alt+M only cycles installed+enabled CLIs
• Manual: repo settings → provider toggles appear with correct install status
• Manual: disable a provider → its models disappear from cycling
• Manual: welcome screen shows provider status line

🤖 Generated with Claude Code

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Note

Medium Risk
Touches session execution/model selection and startup wiring; misconfiguration could prevent sessions from starting or change routing/provider defaults, though changes are guarded by availability checks and covered by tests.

Overview
Adds provider discovery and opt-in controls: Bramble now probes installed AI CLIs (Claude/Codex/Gemini), shows their status on the welcome screen, and lets users enable/disable providers in repo settings; enabled providers filter Alt+M model cycling and persisted settings via new Settings.EnabledProviders.

Refactors AI model/provider plumbing by introducing multiagent/agent ProviderAvailability + ModelRegistry, wiring them through startup and session.Manager to fail fast when a session selects a model whose provider is missing/disabled, and improving tmux-mode completion detection by reading the pane exit status.

Moves the task router into a new bramble/taskrouter package and switches it to the generic agent.Provider interface, with startup selecting the “best available” routing provider (codex→claude→gemini) and updated integration/unit tests.

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[mzhaom]

All review findings have been addressed across multiple commits:

High Severity Issues:

1. Disabling all providers silently re-enables them (eb7a981)

   • Changed var result []string to result := make([]string, 0, ...) so empty slice != nil

2. Missing nil check on router result causes panic (eb7a981)

   • Added nil check before accessing result.Error in Router.Route()

3. Empty enabled providers treated as all-enabled (a6f09ad)

   • Changed EnabledProviders to *[]string pointer type
   • nil pointer = all enabled, non-nil empty slice = all disabled
   • Updated all consumers to check enabledProviders == nil

4. Show() conflates nil and empty (8501188)

   • Fixed Show() to use enabledProviders == nil instead of len(enabledProviders) == 0

Medium Severity Issues:

5. Data race on shared ModelRegistry (eb7a981)

   • Added sync.RWMutex to ModelRegistry
   • Build new slice locally, then swap under write lock

6. Non-installed providers polluting enabled set (a2db38b)

   • Only mark installed providers as enabled in default path
   • Filter out non-installed providers from explicit list
   • Compare against installed count in backward-compat check

7. JSON omitempty loses all-disabled state (a6f09ad)

   • Pointer type *[]string preserves distinction through JSON round-trip

Low Severity Issues:

8. Unused IsProviderEnabled method (a6f09ad)

   • Kept for API completeness, added GetEnabledProviders() helper

9. Duplicated boxWidth calculation (a6f09ad)

   • Extracted dialogBoxWidth() helper to prevent navigation-rendering mismatch

All tests pass and lint is clean. Ready for re-review.