fix: resolve correctness, panic, and UX issues from review

[reckerp]

A batch of fixes surfaced by a full codebase review. One reported item — the
--amend --date=now rewrite — was deliberately left as-is (intended behavior).

HIGH

• ai.rs pipe deadlock — stdin is now written on a separate thread while wait_with_output drains stdout/stderr concurrently. Previously commit --ai with a large staged diff could hang forever once the agent's output pipe buffers filled.

MEDIUM

• commit --amend --ai now diffs the index against HEAD's parent (new get_amend_diff()), so it generates a message from the commit being amended instead of failing with "nothing to commit".
• repo_setup copy_files parser stops at a ] only when it's outside a quoted value — a ] inside a path no longer silently drops later entries.
• repo_setup config round-trip — newlines/tabs/CR are escaped in TOML strings (and \r is decoded), so saved configs parse back correctly.
• File picker & setup file picker now scroll; the cursor could previously move below the fold and off-screen.
• log_viewer routes scroll adjustment through the guarded shared helper, fixing an underflow panic on very short terminals.
• worktree add/remove/rebase/branch-delete pass -- before path/ref positionals to prevent argument injection. (Verified checkout must not get --, so it's untouched.)

LOW

• stash branch extraction locates "on " via ASCII byte-windows on the original string — no more mid-char panic/wrong slice on non-ASCII branch names.
• pr troubleshoot surfaces a non-zero exit from the interactive agent.
• branch/workspace pickers no longer type unhandled Ctrl-combos into the search box.
• recent-commits revwalk is sorted by time (newest first).
• now_secs falls back to epoch instead of panicking on a pre-1970 clock.
• gx setup emits aliases in a stable, sorted order.
• setup script stderr falls back to a duplicated stderr fd when /dev/stderr is unavailable.
• branch picker fetches branch info off-thread, so it no longer freezes on each selection change.

Also dedupes adjust_scroll into ui::adjust_scroll and syncs Cargo.lock to 0.3.0.

Testing

• cargo build, cargo clippy --all-targets, and cargo test all pass (127 tests, including 6 new ones covering the array parser, escaping round-trip, and the stash non-ASCII case).

Note: the repo isn't rustfmt-clean to begin with; only the new code here is formatted, leaving pre-existing diffs untouched to keep this PR focused.