Configurable git status refresh interval to reduce disk I/O in worktree-heavy sessions

[ericma15]

Herdr keeps the sidebar git info (branch name, ahead/behind counts) fresh by polling the filesystem on a hardcoded 1.5s interval (GIT_REMOTE_STATUS_REFRESH_INTERVAL, src/app/mod.rs:38). The loop is active by default: the default sidebar
spaces layout includes branch and git_status tokens (src/config/sidebar.rs:415-424), so git_refresh_demand() is never empty whenever workspaces exist (src/app/git_refresh.rs:102-112).

In worktree-heavy sessions this produces sustained disk I/O:

• Every 1.5s, each workspace triggers a chain of file reads: .git discovery walking up from the cwd (src/workspace/git/discovery.rs:244-262), the linked worktree's own HEAD (src/workspace/git/status.rs:253-269), user and repo git config
  including include chains (src/workspace/git/config.rs:13-55), and loose refs plus a full packed-refs scan (src/workspace/git/discovery.rs:264-287).
• The git status cache is keyed per-checkout repo_root (src/workspace/git/status.rs:68-74), so linked worktrees of the same repository do not share a refresh cycle — N open worktrees means N× the file reads every 1.5s.
• When an agent is active in a worktree (commits, branch switches), the fingerprint changes on each cycle, defeating the cache and additionally spawning git rev-list --left-right --count (src/workspace/git/status.rs:306-325) — plus git
  symbolic-ref / git rev-parse subprocesses on reftable repos (src/workspace/git/discovery.rs:159-169).
• cwd changes reported by panes (TerminalCwdReported) trigger an immediate full discovery refresh (src/app/api.rs:293-297).

The result is a constant stream of reads against the same small set of git metadata files (.git/HEAD, config, packed-refs), scaled by the number of open worktrees.

Requested behavior

1. Make the refresh interval configurable (e.g. a git_status_refresh_interval setting), and/or
2. Reduce the per-cycle cost:
   - key the status cache by the common git dir so linked worktrees of one repo share a single refresh;
   - cache packed-refs and branch config lookups by mtime so unchanged metadata is not re-read every cycle;
   - skip branch-config reads when the status fingerprint is unchanged.

Workaround today

Removing branch / git_status from [ui.sidebar.spaces] rows stops the loop entirely (git_refresh_deadline() returns None), at the cost of losing the sidebar git display.

Additional observations

• Session persistence rewrites session.json plus session-history.json (full pane ANSI scrollback, pretty-printed) on a 5s debounce while the session is dirty (src/persist/io.rs:44-76). With output-heavy panes this is a smaller but
  noticeable secondary write source; a size cap or incremental write would help.