feat: expand stackmap branch workflow

[fieldsofland]

Summary

Stackmap now provides a clearer, safer view of active branch workflows. Custom visual sections and stack summaries organize large Graphite stacks without changing repository topology, while the main view exposes checkout, pull-request, worktree, diff, and local remote-safety state directly on each row.

Highlights

• Add persistent visual section boundaries, white custom titles, inline Unicode-aware editing, section colors, and cumulative name indentation without moving Git/Graphite rails.
• Add true base-to-tip diff summaries for named stacks with branch-first bounded enrichment.
• Add explicit double-Enter checkout confirmation and follow branches whose tips advance after structural refresh.
• Show GitHub PR states including approved, merged, and closed.
• Show no-fetch remote safety badges in Active and Archive views:

Badge	Local evidence
✓ pushed	Upstream is equal or a local remote-tracking ref contains the tip
↑n ahead	Local commits are not represented by the configured upstream
↓n behind	Configured upstream is ahead
↕n/n div	Local and upstream histories diverged
× gone	A configured upstream no longer exists locally
○ no remote	No configured or locally containing remote is known
? remote	Evidence is unavailable

• Keep monitoring passive with optional Git locks disabled, path-filtered filesystem events, bounded workers, and clean subprocess shutdown.
• Add reviewed plans for future agent coordination without enabling unfinished provider integrations.

Remote evidence remains explicitly local and no-fetch. It improves cleanup confidence without declaring a branch automatically safe to delete.

Verification

• cargo fmt --all -- --check
• cargo test --offline --all-targets --all-features (206 tests)
• cargo clippy --offline --all-targets --all-features -- -D warnings
• cargo build --offline --release
• Responsiveness benchmark target
• Installed release artifact matched the verified build checksum