feat(completion): __complete subcommand foundation (#291)

[erishforG]

Summary

• New hidden subcommand: `parsec __complete <worktrees|branches>`
• Emits newline-separated candidates; silent on failure so completion never errors at the prompt
• Adds `git::list_local_branches` helper
• 4 new integration tests (hidden-from-help / branches / worktrees / silent-outside-repo)

Scope of this PR (foundation only)

This is step 1 of 2 for #291. It lands the data-emission half:

```
$ parsec __complete worktrees
PROJ-101
PROJ-102

$ parsec __complete branches
develop
feature/x
main
```

What's NOT in this PR (follow-up)

Step 2 — making the existing `parsec config completions ` output call `__complete` from inside the generated zsh/bash/fish functions. The post-processing approach (replace `_files` with shell-specific `compadd "$(parsec __complete worktrees)"` blocks at known argument positions) belongs in its own PR so this one stays reviewable.

Filing as draft so we can agree on the foundation shape (hidden command name, candidate format, silent-failure policy) before wiring the shell scripts.

Test plan

• `cargo fmt --all -- --check`
• `cargo clippy -- -D warnings`
• `cargo test` — 102/102 pass
• Manual: `parsec __complete branches` / `parsec __complete worktrees` on this repo

Open questions for review

1. Hidden subcommand name — `__complete` (cargo-style) vs `_completion` vs nested under `config`?
2. Should we emit a header like `# worktree-tickets` so shell scripts can sanity-check? (Currently no — keeps parsing trivial.)

Refs: #291

[erishforG]

두 질문에 대한 결정 (현재 코드 유지)

Q1: __complete (cargo style) 유지

• cargo/gh/rustup 모두 동일 패턴. Rust 생태계 표준이라 즉시 이해됨.
• shell script 안에서만 호출되니 발견성 낮은 게 장점.
• config completions(script 생성) vs config complete(값 emit)는 의미가 달라서 같은 그룹에 묶지 않는 게 명확함.

Q2: 헤더 emit 안 함 (현재 그대로)

• silent-failure 정책이 이미 안전망 역할.
• 자동완성은 hot path — 파싱은 while read line 한 줄로 끝나는 게 좋음.
• 잘못된 output 가능성이 낮고, 발생해도 사용자 체감은 "자동완성 안 나옴" 수준 (silent).

→ 코드 변경 없이 ready 전환 + 머지.