Add multi-repo garden windowsill view (#8)

[rwrife]

Closes #8

Adds commit-sprout garden: a multi-repo windowsill that renders a row of small plants, one per repository, so you can see commit cadence across several projects at a glance.

What's here

• internal/gitstat — new ReadAt(dir, author, window) reads a specific repo directory; Read now delegates to it. Each repo defaults to its own git config user.email, so per-repo author filtering is preserved (acceptance criterion). Adds RepoName() for cell labels.
• internal/render — Garden([]GardenPlant, Options) lays out cells side by side, wrapping rows to terminal width, with ANSI-aware width measurement so colored and plain cells pad to the same geometry (columns always line up). Reuses the existing per-stage/health art.
• internal/store — persists a saveable garden repo set (garden_repos), fully additive/backward-compatible, with AddGardenRepos/RemoveGardenRepos (absolutize + de-dup).
• cmd/garden.go — commit-sprout garden [paths...], --scan <dir> (one-level parent scan), saved-set fallback, --add/--remove/--list management, --author/--window. Read-only render: it never mutates any repo's remembered plant state.

Acceptance criteria (#8)

• Config lists multiple repo paths (or a parent dir to scan) — --add/saved set and --scan
• commit-sprout garden renders one small plant per repo side by side (the "windowsill" variant)
• Per-repo author filtering still applies (each repo uses its own user.email; --author overrides)

Example

foo                  bar
   \|/                  ,
    |                  /|
   _|_                ' |
  (   )                _|_
   '-'               (   )
seed/healthy          '-'
no commits yet     leafy/healthy
                   today (2026-07-06)

Resolution order

explicit path args → --scan <dir> → saved set.

Testing

gofmt -l clean, go vet ./... clean, go build ./... ok, go test ./... -race -count=1 green (new tests in gitstat/render/store/cmd covering row packing, wrap-at-width, ANSI-safe padding, add/remove/round-trip persistence, scan/precedence). Manually smoke-tested add/remove/list/render + color alignment + bad-path skip.

README updated with a Garden section and roadmap note.