fix(taskctl): detect default branch dynamically instead of hardcoding 'dev'

[randomm]

Problem

taskctl start hardcodes dev as the base branch in two places:

1. packages/opencode/src/tasks/job-commands.ts:84 — git checkout -b feature/issue-NNN dev
2. packages/opencode/src/tasks/pulse-scheduler.ts:155-190 — git rev-parse --verify dev and git merge-base dev HEAD

This breaks taskctl start in any repo whose mainline is not named dev (e.g. main, master, trunk).

Fix

Detect the default branch dynamically using:

git symbolic-ref refs/remotes/origin/HEAD --short
# → e.g. "origin/main" → strip prefix → "main"

This pattern already exists in packages/opencode/src/worktree/index.ts. Extract it into a shared utility and use it in both executeStart and pulse-scheduler.ts.

Fallback chain if detection fails:

1. git symbolic-ref refs/remotes/origin/HEAD --short
2. git remote show origin | grep 'HEAD branch'
3. 'dev' (last resort)

Quality Gates

• Tests written (TDD preferred)
• Coverage meets project threshold
• Linting passes
• Local verification complete