fix(worktree): reconcile detached-HEAD worktrees, run prune on clean, clamp zero timeout

[bug-ops]

Summary

Three related bug fixes in crates/zeph-worktree, spec-governed by specs/063-worktree-subsystem/spec.md and srs.md:

• WorktreeManager::reconcile() silently dropped detached-HEAD worktrees because the git worktree list --porcelain parser only flushed a block when both a worktree <path> line and a branch line were present. Detached worktrees emit detached instead of branch, so they became permanently invisible to zeph worktree list/zeph worktree clean. The parser now flushes whenever worktree <path> is seen, and assigns a new DETACHED_BRANCH_SENTINEL ("(detached HEAD)") as branch_name when no branch line is present. The sentinel contains a space, which git check-ref-format --branch rejects, so it can never collide with a real branch name — including on a worktree foreign to zeph, since reconcile() enumerates all worktrees in the repo, not just zeph-managed ones.
• zeph worktree clean never called git worktree prune after removing stale entries, per spec-063 FR-CLEANUP-04. Added WorktreeManager::prune() and wired it into the clean command after the removal loop.
• DefaultGitRunner did not clamp git_timeout_secs = 0 itself; the clamp was duplicated ad hoc at two call sites, and the crate's derived Default impl bypassed both (zero-initializing the Duration field). The clamp now lives in DefaultGitRunner::new()/with_timeout(), the ad hoc call-site clamps were removed, and Default is now a manual impl delegating to new().

A P3 follow-up issue will be filed separately for bare-repo worktrees also falling through to the detached sentinel (mislabeling, not a spec violation — spec-063 doesn't cover bare worktrees) — that gap was found during review and intentionally deferred out of this PR's scope.

Closes #5936
Closes #5937
Closes #5939

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 12828 passed, 0 failed
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• RUSTFLAGS="-D warnings" cargo check --workspace --all-targets --features desktop,ide,server,chat,pdf,scheduler --locked
• cargo test --doc --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• New real-git integration tests in crates/zeph-worktree/tests/real_git_integration.rs (uses the real git binary, not a fake runner): prune clears a manually-removed worktree's leftover admin entry, full clean pipeline (reconcile+remove+prune), zero-timeout clamp against a real git invocation, and the sentinel's ref-name invalidity via git check-ref-format
• .local/testing/playbooks/worktree.md and .local/testing/coverage-status.md updated in the main repo