feat: less pager mode with smooth Kitty image scrolling and Mermaid theming

[kkovary]

Summary

Adds a less-style pager, Mermaid theming, and a live-reload watch mode, and makes line-by-line scrolling + terminal-aware theming the defaults.

Pager: new less mode (now the default)

• Scrolls a line at a time (j/k, arrow keys, mouse wheel) instead of more's page jumps, plus half-page (d/u), page (space/b), g/G top/bottom, and / ? n N search.
• On Kitty-graphics terminals (Kitty, Ghostty), inline Mermaid diagrams scroll smoothly: scroll position is tracked in display rows and each frame emits a vertically-cropped Kitty placement for the visible slice of an image, so diagrams scroll partially off-screen like text instead of popping in/out. iTerm2/sixel keep the existing atomic behavior.
• Redraws use synchronized output (DEC mode 2026), with input coalescing and no-op-repaint skipping, to eliminate flicker on fast key-repeat and mouse scrolling.

Mermaid: mermaid.theme config (default auto)

• Applies a Mermaid theme to inline images and --pdf export: auto, light/default, dark, forest, neutral, base.
• auto queries the terminal background via OSC 11 and renders dark diagrams on a dark background, light otherwise. Falls back to light when the background can't be determined (e.g. PDF export to a file).

Watch mode (--watch)

• Watches the input file and refreshes the pager in place when it changes, preserving scroll position — for an edit-in-one-pane / preview-in-another workflow.
• Polls mtime+size with a debounce (coalescing rapid saves). The pager loop becomes event-driven: a goroutine feeds terminal bytes to a channel and the loop selects over keystrokes + a reload signal, so the view refreshes without a keypress. Diagrams re-render on change.
• --watch / --no-watch flags and a pager.watch config option, off by default. Gated to a file argument rendered to a TTY; otherwise renders once with a note.

Defaults & docs

• Default pager.mode is now less (was more); default mermaid.theme is auto.
• README documents pager.mode, pager.watch, and mermaid.theme.

Behavior change

The default pager changes from more to less. Users who prefer the old behavior can set {"pager": {"mode": "more"}}. vim mode is unchanged.

Testing

• New unit tests: Kitty crop pixel math, row-scroll/segment math and search, OSC 11 color parsing + luminance threshold, theme resolution, watcher change-detection + debounce, reload scroll preservation, and config defaults.
• go build ./..., go vet ./..., and the full go test ./... suite (including the Chrome-backed Mermaid tests) pass, plus -race on the watch/pager packages.

Notes

• Smooth image scrolling and watch require the Kitty graphics protocol for inline diagrams; other terminals fall back gracefully.
• The first inline-diagram render adds a one-time ~200ms terminal background query (only when a document contains diagrams).
• Watch detects the terminal background via stdin and is gated to a file argument, so cat foo.md | glowm --watch does not watch.