fix(auth): clearer import-brave error + add to README/skill docs

[jprichardson]

Summary

• import-brave silently failed on first run because extractFromBrave() wrapped its body in try { ... } catch { return null }, hiding the specific "Allow JavaScript from Apple Events is turned off" failure behind a generic "ensure you're logged in" message.
• That toggle ships OFF in Brave (and every Chromium browser), so the very first agent-slack auth import-brave consistently looks like an auth problem instead of a one-time browser setting.
• Adds import-brave to README + skill docs, where it was previously undocumented despite being wired into the auto-detection fallback chain (src/cli/context-client-resolver.ts:138).

What changed

• src/auth/brave.ts: detect the AppleScript-disabled error in osascript() and throw a typed BraveAppleScriptDisabledError with an actionable message (menu path + re-run hint). Other failure modes keep the existing null-return behavior, so the generic "ensure you're logged in" message still applies when no Slack tab is open or cookie decryption fails.
• src/cli/auth-command.ts: import-brave's description and fallback throw new Error message both reference the View → Developer → Allow JavaScript from Apple Events requirement.
• README.md: list import-brave in the command tree and manual-imports block; add a NOTE block explaining the AppleScript-JS prereq applies to both import-brave and import-chrome.
• skills/agent-slack/SKILL.md: add a Brave fallback section and update the intro to Brave/Chrome/Firefox (Brave is already in the auto-fallback chain).
• skills/agent-slack/references/commands.md: add import-brave to the auth command list with the AppleScript prereq inline.

Notes

src/auth/chrome.ts has the same bug pattern (identical try { ... } catch { return null } wrapping + identical execute t javascript AppleScript path). Out of scope for this PR — happy to send a follow-up.

Test plan

• bun run typecheck clean
• bun run test — 198/198 pass
• bun run format:check clean
• agent-slack auth import-brave happy path: still imports tokens (2 workspaces in my case)
• Manually reproduced AppleScript-disabled failure (View → Developer → Allow JavaScript from Apple Events OFF) and confirmed osascript() reports the exact marker string that the new detection relies on
• CI

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