feat: filter PRs by merge state + help/legend overlay

[sashml]

Summary

Adds a merge-state filter (f) and a help overlay (?) with the State/CI glyph
legends — the final piece of the merge-state visibility epic (#1).

• f cycles all → conflicting → clean; active filter shown in the table title and
  composes with / search.
• ? opens a help overlay documenting every keybinding + both glyph legends (CI-5).

⚠️ Stacked on #7 (→ #5 → #4)

Last in the cascade. Until the earlier PRs merge, the diff shows all four commits;
review/merge after #4, #5, #7. Only the last commit belongs to this PR.

Changes

• state: MergeFilter {All, Conflicting, Clean} applied in filtered_prs (one place →
  every view); help_open flag; Esc/h close overlays
• event_loop: f → cycle filter, ? → toggle help
• ui: table-title filter indicator; render_help_overlay; status-bar hints updated
• tests: cycle order, help toggle, back-closes-help, filter selection + search composition

Testing

cargo build, cargo test (8 binaries; state_tests 38), cargo fmt --check,
cargo clippy — clean (no new warnings).

Closes #10
Part of #1

[sashml]

@zombocoder requesting your review 🙏 — final piece (filter + help/legend). Cascades on #7 (→ #5 → #4); please review/merge last.

[sashml]

Rebased onto current main and reconciled with the git-log/diff overlay rework (#9). This branch is now a single commit on top of main:

• f cycles the merge filter (all → conflicting → clean); composes with / search; active filter shown in the table title.
• ? opens a help overlay documenting all keys (incl. l log / d diff) + the State/CI glyph legends.

The filter/help are additive and independent of the per-PR Overlay state, so no conflict with #9. Verified locally: cargo test (all pass), cargo fmt --check, and RUSTFLAGS=-Dwarnings cargo clippy all clean — the earlier Clippy failure (CI uses -Dwarnings) is resolved.

@zombocoder ready for another look 🙏