docs: thank outside contributors on their pull requests

[michael-denyer]

What and why

• Add AGENTS.md. When a pull request from a fork merges, the agent comments to thank the author by handle, says what the change fixed, and links any follow-up pull requests. A fork pull request closed without merging gets thanks and a reason. fix(pi): map the anthropic haiku family to claude-haiku-5-5 #252 merged with no thank-you until a later comment, and this makes the step standard.
• The release entry in CHANGES.md credits the author by handle, as the 0.9.80 entry does for fix(poteto-mode): observe current base branch targets #245. The file records that existing practice next to the new one.
• AGENTS.md points at CONTRIBUTING.md for commands, checks, and release steps and does not copy them. No CLAUDE.md exists in the repository or above it, so Claude Code reads AGENTS.md.

Linked issue

None.

Test run

npx --yes markdownlint-cli2@0.18.1 AGENTS.md
    0 errors
bun tools/generate.mjs --check
    exit 0
bun test
    1191 pass, 36 skip, 1 fail. The failure is tests/pi/catalog.test.mjs against this Mac's global
    Pi 1.0.0 (pi-ai 1.0.0), which is below the Pi 1.1.0 floor that #255 set. The test is unrelated to AGENTS.md.
PI_PACKAGE_DIR=<npm Pi 1.1.0> PSTACK_PI_REQUIRE_CATALOG=1 bun test tests/pi/catalog.test.mjs
    1 pass, 0 fail

Checklist

• I ran bun install --frozen-lockfile, bun tools/generate.mjs, and bun test tests/, and committed the generator's output.
• bun tools/generate.mjs --check exits 0.
• I changed model defaults only in plugins/pstack/models.json, and every Pi model ID is in Pi's catalog for that provider.
• If this changes skill behavior, it adds a CHANGES.md entry and bumps VERSION, or names the release PR that will.
• If this touches plugins/pstack/pi/, pi-tools.md, plugins/pstack/hooks/, copilot-tools.md, or setup-pstack, I ran the runtime checks that CONTRIBUTING.md lists for those paths.

[michael-denyer]

Closing: AGENTS.md stays local and untracked in this repository.