docs: add operations + rotation playbooks, recipes, limits, API toolkit

[hasansezertasan]

Summary

Expands the docs/ layer from "what this is" to "how to run it" with seven additions and an index. The big swing is moving from first-contact orientation (covered by #9) into operational reference: when something breaks, when something needs adding, when a token rotates, what each vendor limit is, and how to drive HCP from CLI.

What's new

File	Purpose
docs/rollback.md	Six ranked options when apply goes wrong: revert, discard, cancel, state surgery, admin override, rebuild.
docs/recipes.md	Copy-paste recipes for the five tasks future-you will do most often: add DNS record, add repo, add label, bump provider, cross-workspace change.
docs/limits.md	Free-tier ceilings on HCP, Cloudflare, GitHub, Let's Encrypt with current usage and cliffs to watch.
docs/hcp-api.md	REST API toolkit. Every `curl` we used during the bootstrap, generalised and reusable.
docs/worktree-workflow.md	Optional `git worktree` convention for maintainers juggling branches.
docs/README.md	Index of every doc with one-line hooks and a reading order.

What's extended

• `docs/secrets.md` Rotation section: from one-line-per-credential into step-by-step zero-downtime procedures with "no plan, no revoke" verification.
• `terraform/README.md` Conventions section: lock file committed, label naming, file-per-category split, provider pin policy (`~> X.Y`), cross-leaf references hedge.

Test plan

• All cross-references between docs resolve.
• No conflicts with docs: add CONTRIBUTING.md as 5-min onboarding for human or AI #9 (PR docs: add CONTRIBUTING.md as 5-min onboarding for human or AI #9 touches `setup.md` and `import.md` only; this PR doesn't touch those).
• HCP rolls up SUCCESS (docs-only PR).
• Reader smoke test: a fresh contributor reading `docs/README.md` can find any of the seven operational topics by skim alone.

Pairs with

• docs: add CONTRIBUTING.md as 5-min onboarding for human or AI #9 — CONTRIBUTING.md (first-contact orientation).
• feat(hcp): manage HCP workspaces, project, VCS connection via the tfe provider #8 — HCP-as-code arc (the docs added here are what the next session will lean on for that work).

After this lands, the docs/ surface is opinionated, indexed, and operationally complete enough that the next agent can pick up issue #8 cold with full context.