fix(ai-tools): honor defaultAI config in termly start

[kozliatko]

Problem

defaultAI is currently a write-only config key.

Three code paths write it, none read it:

Location	Role
lib/config/manager.js:7	defines the schema
lib/commands/setup.js:30	termly setup stores the answer
lib/commands/config.js	termly config set defaultAI <tool> stores it, termly config prints it
lib/config/manager.js:44	getDefaultAI() — exported, zero call sites

grep -rn "getDefaultAI" lib/ on main returns only the definition and the export.

The user-visible effect: someone runs termly setup, answers "Default AI tool: claude-code", sees Default AI: claude-code in termly config — and still gets the interactive Multiple AI tools detected: Which tool would you like to use? prompt on every single termly start. The setting silently does nothing.

Fix

selectAITool() now consults the config between the --ai flag and auto-detection:

--ai flag  ->  defaultAI config  ->  --no-auto-detect error  ->  auto-detect

--ai still wins, so nothing changes for explicit invocations.

Stale values are non-fatal

Unlike --ai, defaultAI is a stored preference that may have been set months ago. Routing it straight through selectManualTool() would have made termly start exit 1 the day a user uninstalls or renames their configured tool.

So selectConfiguredDefault() warns and returns null, letting the caller fall back to auto-detection:

⚠️  Configured default AI Aider is not installed - falling back to auto-detection
   Change it with: termly config set defaultAI <tool>
✓ Using Claude Code v2.1.263 (auto-detected)

Behavior change

Users who already have defaultAI set — possibly without realizing, since it never did anything — will stop seeing the tool-selection prompt. That is the behavior the setting always advertised, but it is a change worth calling out in release notes.

Verification

All four paths exercised against a stubbed getDefaultAI():

Scenario	Result
--ai claude-code + defaultAI=demo	claude-code — flag wins
defaultAI=demo (installed)	Using Demo Mode (configured default)
defaultAI=nonexistent-xyz	warns, falls back to auto-detect
defaultAI=aider (not installed)	warns, falls back to auto-detect
defaultAI=''	auto-detect, unchanged

Scope: lib/ai-tools/selector.js (+41) and a README note (+4). No dependency, protocol, or config-schema changes.