Fix #580 PackV2 naming and warning cleanup

[julianknutsen]

Summary

• align PackV2 docs, schema, and runtime guidance to the current agent_defaults and prompt-template contract
• preserve imported pack defaults and warning plumbing across config loading, CLI surfaces, and import rewrite paths
• stop transitive = false imports from leaking nested providers, globals, requirements, services, and formula layers
• add consistent v0.15.0 pre-release caveat banners and release-qualified wording across the PackV2 docs touched by this rollout

Follow-up

• track deferred agent_defaults surfaces removed from the tightened docs in Track deferred PackV2 agent-defaults surface removed from current docs #903 (provider, scope, install_agent_hooks)
• keep agent-local append_fragments tracked separately in Pack v2: resolve agent-local append_fragments spec/runtime gap #671

Testing

• go test ./internal/config
• go test ./internal/config -run '^(TestImport_TransitiveFalseSuppressesNestedCityImportArtifacts|TestImport_TransitiveFalseSuppressesNestedPackImportArtifacts|TestImport_RigImportTransitiveFalseSuppressesNestedDeps|TestImport_TransitiveFalseSuppressesNestedNamedSessions|TestImport_TransitiveFalseSuppressesLegacyIncludedPackDeps|TestImport_TransitiveFalseSuppressesNestedPackWarnings|TestParseWithMetaSkipsMixedTableWarningWhenCanonicalAndAliasAreDisjoint|TestParseWithMetaSkipsMixedTableWarningWhenOverlapIsOnlyUnsupportedFutureKeys)$' -count=1
• go test ./cmd/gc -run '^(TestStrictFatalLoadConfigWarningsKeepsMixedTableWarningsFatal)$' -count=1
• prior Claude branch review major fixed; fresh Codex rerun on the updated diff: NO BLOCKER OR MAJOR FINDINGS.

[csells]

I like that the individual areas of the docs say things like "in this release" or "in this wave". I'd like to see two things change:

• At the top of every page with a release-specific qualifier, I'd like to see a banner that says something like "This document covers the pre-release version of Gas City v and therefore some of our features are still under active development. You can read the details below." This banner should be clear for the user to see are a separate style from the rest of the docs on the page.

• I'd like to see each caveat currently marked with "in this release" or "in this wave" or even "in this release wave" to be standardized to be "In this release, ...". Even better if it could easily say "As of release v, ...".

[csells]

I'm not seeing the things I requested yet in this PR.

[csells]

"In this release"

[julianknutsen]

Addressed the outstanding docs feedback on this branch:

• added a consistent pre-release v0.15.0 banner to each release-qualified PackV2 doc touched here
• normalized the inline caveat wording to use explicit release-qualified phrasing
• created follow-up issue Track deferred PackV2 agent-defaults surface removed from current docs #903 for the deferred agent_defaults surface that was removed from the tightened current docs

The only code changes after the earlier review loop were the nested transitive = false import fix and these docs updates.

[julianknutsen]

Merge Conflicts Detected

GitHub reports that this PR currently has merge conflicts with main. Because maintainer edits are disabled on this PR, we cannot rebase or push the conflict resolution to your branch from the adoption workflow.

Please rebase your branch against main and force-push to update this PR:

git fetch upstream main
git rebase upstream/main
# resolve conflicts
git push --force-with-lease

We'll re-review once the conflicts are resolved.

[julianknutsen]

Adoption summary for d68843751646f4e853b0b85ee1eca913804071bd:

• Rebasing onto current main required conflict resolution across pack config loading, warning emission, CLI config callers, and integration fixtures.
• Added a fixup commit to preserve fatal unknown-field validation for pack.toml while keeping migration/provenance warnings non-fatal, and cleaned up the remaining lint issues found by CI.
• Local validation included make lint, go vet ./..., the affected package tests after the final rebase, and a full go test ./... -count=1 -timeout=20m before the final clean rebase. GitHub CI is green on the pushed head.

Proceeding with squash merge.