docs: add README, CHANGELOG, NOTICE, SECURITY, and LLM docs bundle

Summary

Land the user-facing documentation bundle modelled on axonops/mask: a full README.md, CHANGELOG.md, NOTICE (crediting the original rgooding/go-syncmap author), SECURITY.md, llms.txt, and llms-full.txt (with the scripts/gen-llms-full.sh script that regenerates the latter). Re-enables the markdownlint and llms-full-up-to-date CI guard jobs that were deferred in #8.

Requirements

1. README.md — rewrite from the current 1-line placeholder. Structure mirrors mask's README. Sections:
   • Logo / tagline (use gopher.png at repo root if it passes markdownlint image-alt requirements)
   • Project description
   • Installation: go get github.com/axonops/syncmap
   • Quick Start (copy-paste-runnable complete program)
   • API Reference (every public symbol — signature + one-sentence description + link to godoc)
   • Thread Safety (inherits from sync.Map)
   • Performance (link to bench.txt and summarise overhead vs raw sync.Map)
   • When to Use (SyncMap vs sync.Map vs map+sync.RWMutex)
   • Contributing (link to CONTRIBUTING.md)
   • Attribution (fork of github.com/rgooding/go-syncmap by Robert Gooding)
   • Licence
   • CI badge(s) matching mask's README
2. CHANGELOG.md — Keep-a-Changelog structure. Seed with the v1.0.0 entry listing every change from the fork: module rename, Go 1.26, method renames/additions, CI/release setup, docs, tests. Future versions documented as they cut.
3. NOTICE — Apache 2.0 attribution block that names AxonOps Limited AND credits Robert Gooding / github.com/rgooding/go-syncmap as the original source. Wording modelled on mask's NOTICE.
4. SECURITY.md — mirror mask's SECURITY.md exactly, substituting mask → syncmap in the reporting URL / email as appropriate.
5. llms.txt — mirror mask's llms.txt structure (common mistakes AI assistants make; project overview for LLM context).
6. llms-full.txt — generated concatenation of the policy-bearing docs; produced by scripts/gen-llms-full.sh.
7. scripts/gen-llms-full.sh — mirror mask's script verbatim (adjust source file list to syncmap's docs).
8. Makefile — re-add the llms-full and llms-full-check targets (removed in chore: align CI, release, and tooling to axonops/mask pattern #8). Update makefile-targets-guard expected list accordingly.
9. CI:
   • Re-enable markdownlint job in ci.yml (uses DavidAnson/markdownlint-cli2-action@v21.0.0 per mask pin) and list README.md, CHANGELOG.md, CONTRIBUTING.md, SECURITY.md, NOTICE, CLA.md, CODE_OF_CONDUCT.md, CONTRIBUTORS.md (the ones that exist by merge time) in its globs: input.
   • Re-enable llms-full-up-to-date job in ci.yml.
10. All new files carry appropriate licence / attribution. No AI-attribution tokens outside the llms files (where tool names are product references, already excluded by attribution-guard).

Acceptance Criteria

1. README.md ≥150 lines; contains every section listed in Req 1.
2. CHANGELOG.md has a populated ## [1.0.0] section.
3. NOTICE names both AxonOps Limited and Robert Gooding (rgooding/go-syncmap).
4. SECURITY.md exists and documents private disclosure.
5. llms.txt and llms-full.txt exist at repo root.
6. scripts/gen-llms-full.sh is executable and idempotent.
7. make llms-full regenerates llms-full.txt without diff.
8. make llms-full-check exits 0 on an unchanged tree.
9. markdownlint CI job passes on this PR.
10. llms-full-up-to-date CI job passes on this PR.
11. makefile-targets-guard passes with llms-full and llms-full-check re-added.
12. make check green; CI green end-to-end.
13. grep -i 'rgooding/go-syncmap' NOTICE CHANGELOG.md README.md returns at least one hit in each.

Testing Requirements

• go test -run 'Example' ./... — if examples exist, they continue to pass (this issue adds prose, not test code).
• CI — all green including the two re-enabled guards.
• markdownlint-cli2 locally against the doc file list — zero warnings.
• make llms-full-check on unchanged tree — exit 0.

Documentation Requirements

• This IS the documentation issue.

Dependencies

• Blocked by: docs: add doc.go and rewrite godoc on every exported symbol #11 (godoc — README's API Reference quotes it), chore: align CI, release, and tooling to axonops/mask pattern #8 rename (if it merged — else README carries the old name), chore: align CI, release, and tooling to axonops/mask pattern (#8) #9/test: rewrite unit tests to external package with testify/goleak; coverage ≥95% #10 (README's API Reference table), feat!: rename Items to Values #12 (bench.txt must exist so README can link to it).
• Blocks: v1.0.0 — no release without README.

Labels

• documentation
• P0