feat: one-time GitHub-star nudge (postinstall + doctor) + RepoStars README badge

[ken-jo]

What / Why / How

Adds a friendly, consent-gated GitHub-star nudge — shown at most once after a real global install and once after context-mode doctor — plus the RepoStars badge at the bottom of the README. (Feature request, not a bug — no Fixes #.)

With this PR applied, after a real npm i -g context-mode and after a context-mode doctor run, the user sees a single star request; the account is starred only on an explicit "Y".

This PR (one commit per item):

• star nudge — src/star-nudge.ts maybeStarNudge(surface), wired into src/cli.ts doctor() (interactive consent) and scripts/postinstall.mjs (print-only). Shown once per surface via marker ~/.context-mode/.star-nudge.<surface>; only when gh is present and the repo is not already starred; stars only on an explicit "Y" at the Star now? (Y/n/skip) prompt; no TTY (postinstall) → message + URL only, never stars; opt out with CONTEXT_MODE_NO_STAR_NUDGE; never under CI; best-effort throughout.
• README — RepoStars badge at the bottom.
• tests — tests/star-nudge.test.ts: opt-out, CI-safety, and the once-per-surface marker guard.
• preserved maintainer invariants: opt-OUT env only (no opt-in flag); exact string compare for the prompt (no NL regex); no MCP/adapter/hook/session behavior changed.

Affected platforms

Agent / CLI scope:

• All platforms — this is the shared install (postinstall) + context-mode doctor CLI layer, common to every agent; no agent-specific or MCP/adapter code path is touched.

OS scope:

• Linux — build + typecheck + unit tests + fake-gh smoke.
• macOS / Windows — not separately gated: star-nudge.ts is pure cross-platform Node (os.homedir(), spawnSync, readline, fs, path.join) with no platform-specific branches, and the postinstall hook is OS-agnostic. (A macOS spot-smoke also passed.)

Scope notes:

• Touches only the install/update + CLI doctor path. No change to the MCP server, hooks, adapters, or any agent integration — Claude Code / Codex / Cursor / Gemini / pi / OpenCode runtime behavior is unaffected.

Root Cause

N/A — this is an additive feature, not a fix for prior behavior, so there is no regression source to trace. It introduces one new module and two call sites and preserves the maintainer invariants listed above (opt-out-only env, no NL regex, no shared-path behavior change).

Validation

Check	Result
npm run build + npm run typecheck	pass
vitest run tests/star-nudge.test.ts	3 pass (opt-out, CI-safety, once-per-surface marker)
Smoke (fake gh, not starred, non-TTY)	nudge prints once → marker suppresses repeat ✓
Smoke (already starred)	silent; marker written ✓
Smoke (gh absent)	no-op; no marker (retries later) ✓
Smoke (non-TTY)	prints message; never prompts / stars / hangs ✓
context-mode doctor (fake gh)	exits 0, nudge appended after the outro ✓

Behavioral evidence:

• BEFORE: no star prompt anywhere.
• AFTER: a single, consent-gated prompt after a real global install and after doctor; never stars without an explicit "Y"; never runs in CI or under the opt-out env.

Cross-agent / cross-OS risk

Minimal. The change lives entirely in scripts/postinstall.mjs and the context-mode doctor CLI path; it adds no dependency and does not touch the MCP server, hooks, adapters, or session paths. Pure cross-platform Node, so no OS-specific risk. The only outward action (gh api -X PUT) runs solely on explicit user consent at an interactive prompt.

Checklist

• Tests added with a real behavioral regression guard (opt-out, CI-safety, once-per-surface marker)
• New test file justified — new star-nudge module; no existing focused test owns this behavior
• npm run build + npm run typecheck pass
• Generated bundles regenerate via CI (not committed)
• No new runtime dependency
• One PR, one commit per item (star nudge; README badge)
• Env is opt-OUT only (CONTEXT_MODE_NO_STAR_NUDGE) — no opt-in flag
• No regex for prompt parsing — exact string compare
• 3OS validation — Linux verified; macOS/Windows not separately gated (OS-portable Node, no platform branches)
• Targets next