docs: update README and docs for v0.18.0

[gtsiolis]

Summary

Audited README.md, CLAUDE.md, every internal/*/CLAUDE.md, and every file under docs/ against current source for the v0.18.0 release, covering the full range since the last docs update (docs: update README and docs for v0.17.1, #389).

Most of the delta since then was already documented in-PR (--type flag, LSTK_MERGE_STRATEGY, SIGINT/SIGTERM forwarding, the bash-completion fix, the emulator-selection fix). Cross-checking lstk --help output, the Makefile, and source against the docs turned up a few real gaps:

• CLAUDE.md: make build no longer cleans first (changed by Fix test-integration running against a stale binary #395, which switched the target to a plain go build relying on Go's build cache) — the "Build and Test Commands" comment still claimed otherwise. Also added the internal/proc package (added by feat: forward SIGINT/SIGTERM to wrapped tools for graceful shutdown #391) to the Architecture package list; it was referenced elsewhere in the file but missing from the list itself.
• README.md: documented the -n/--tail flag added to lstk logs (Add -n/--tail support to lstk logs #359) in the Features list and a Usage example; added the LSTK_MERGE_STRATEGY env var to the Environment Variables table (Add LSTK_MERGE_STRATEGY env var for snapshot load default #400); noted that lstk start --timeout overrides LSTK_STARTUP_TIMEOUT for a single run; extended the "Passing environment variables to the container" section to describe the vars filterHostEnv now drops (and warns about) instead of forwarding — LOCALSTACK_AUTH_TOKEN, and any host var whose stripped name would clobber a critical container var like PATH/HOME (fix: don't let LOCALSTACK_PATH clobber emulator PATH #378).
• internal/container/CLAUDE.md: added a "Host environment forwarding" section documenting filterHostEnv's drop rules (silent for LOCALSTACK_AUTH_TOKEN, warned for newline-containing values and critical-var clobbers), since this logic had no reference doc anywhere.

Needing a human decision

None — no public-messaging wording changed; all additions describe existing, already-shipped behavior.

Test plan

• make build succeeds
• Cross-checked lstk --help and lstk logs --help output against the new README content
• Verified the filterHostEnv drop rules against internal/container/start.go
• Verified the Makefile's build/test-integration targets against the corrected CLAUDE.md wording
• Docs-only change; no other build/test impact

Co-Authored-By: Claude <noreply@anthropic.com>

[gtsiolis]

Heads-up: Probably not as relevant after we merge #406, may have to revisit or close.

[gtsiolis]

Good catch — checked #406's diff: it removes the README's Features list, Environment Variables table, "Passing environment variables to the container" section, and Usage examples entirely (in favor of linking out to docs.localstack.cloud), which is exactly the content three of my four changes here touch.

The CLAUDE.md and internal/container/CLAUDE.md changes (the stale make build note, the missing proc/ package, and the new "Host environment forwarding" reference section) are unaffected either way — those aren't part of the README trim.

Plan: once #406 merges, I'll rebase this PR and drop the now-nonexistent README hunks, keeping only the CLAUDE.md fixes. Let me know if you'd rather I just close this one and let a future docs-sync pass pick up the CLAUDE.md fixes fresh instead.

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