Add .obvious/ agent onboarding contract

[obvious-autobuild]

Onboarding run — sightmap/atlas

Autobuild onboarding: local dev stood up and verified end-to-end, sandbox snapshot captured, .obvious/ contract written. This PR touches only .obvious/ — no entries/ paths, so entry validation skips cleanly per validate.yml.

Files generated

• .obvious/obvious.md — repo guide: stack, commands, codebase map pointer, local verification + Validation Summary, snapshot info (75 lines)
• .obvious/codebase-map.md — depth-2 folder map, single table
• .obvious/config.yml — policy auto-detected from GitHub settings: default branch main, merge method squash (the only method enabled), requireHumanMerge: true
• .obvious/skills/local-dev/SKILL.md — durable bring-up + verification record (name: local-dev frontmatter)

Source files scanned

package.json, package-lock.json, README.md, CONTRIBUTING.md, docs/SPEC.md, docs/POLICY.md, skills/map-a-site/SKILL.md, scripts/validate-entry.mjs, scripts/gen-index.mjs, scripts/lib/front-matter.mjs, scripts/lib/sightmap-cli.mjs, scripts/dev/make-screenshot.mjs, schema/entry.schema.json, .github/workflows/{validate,publish,comment}.yml, .gitignore, plan/IMPLEMENTATION.md, removed.yaml, all 14 entries/*, fixtures/*

Local dev validation result

dev_stack_healthy: true

• npm ci — clean install (after removing a root-owned node_modules left by sandbox provisioning)
• Sightmap CLI built from the repo pin: go install github.com/sightmap/sightmap/go/cmd/sightmap@main (resolved v0.31.1)
• Primary flow: node scripts/validate-entry.mjs entries/github → PASS, 10/10 checks
• Full suite: 14/14 entries PASS (airbnb … zillow); fixtures/invalid-entry → FAIL (6 checks) as designed — the negative path
• node scripts/gen-index.mjs → wrote index.json (14 entries, stats via sightmap stats), exit 0, gitignored exactly as SPEC requires
• Note: this repo has no separate lint/typecheck/test npm scripts — the validate pipeline is its check suite (recorded in the contract; not treated as a blocker)

Sandbox snapshot

• Captured 2026-09-03T17:32:04.893Z — snapshot ID ism0498p7p2tbx2t81u8h (baked as template buipnamg8rvuxmata2i4:default)
• Baked in: npm node_modules, Go 1.24.4, built CLI at ~/go/bin/sightmap, freshly generated index.json

Blockers

None.

Explicit omissions

• fixtures/valid-entry fails sightmap-validate/sightmap-lint against CLI main: it was adapted from an older spec example and uses selector-style extract modes (extract: h2, extract: li:first-child) plus a transform: field the current CLI rejects. Pre-existing fixture-vs-CLI drift — the same binary passes all 14 real entries. Left untouched and flagged for maintainers (it also affects the CLI-bump verification procedure documented in validate.yml).
• Untracked provisioning artifact bun.lock left as-is and not staged (npm is canonical per CI).
• No root AGENTS.md created or modified; no .obvious/tools/, .obvious/system-prompt.md, or .obvious-install/ written.