feat: establish Nx migrations foundations

[nx-plugin-for-aws]

Reason for this change

First step of the Nx Migrations workstream for v1.0 (#718): before any migration can ship, the plugin needs migrations.json wired in, a versioning mechanism compatible with the repo's tag-only release model, and documented contributor guidelines. Strategy discussed and locked in on #703 (deterministic vs agentic split per Nx 23's agentic nx migrate).

Description of changes

• Wire migrations.json into packages/nx-plugin/package.json via the nx-migrations field, and copy it into dist as a compile asset. The collection starts empty — no released change currently warrants a migration (the feat(ts#mcp-server): stop vending a nested project package.json #916 nested package.json removal leaves a harmless leftover in existing workspaces, which per the strategy is not a migration).
• Version stamping at package time (scripts/stamp-migrations.ts, new stamp-migrations target in the package dependency chain). Source migrations.json entries carry no version field — the release model never commits versions to source. The script derives them from git release tags:
  • a migration present in a previous tag keeps the version of the earliest tag registering it (never re-runs for users already past that release);
  • an unshipped migration is stamped strictly between the latest tag and any possible next release (e.g. 1.0.0-rc.32 → 1.0.0-rc.32.1), so it runs for every user upgrading from any released version regardless of what the next release number turns out to be. Semver precedence comparison is implemented in utils/migration-versions.ts (git tag sort isn't semver-aware for prereleases).
• Contributing guide: new Migrations section in CONTRIBUTING.md covering the three Nx 23 migration kinds (deterministic implementation / agentic prompt / hybrid), what should and shouldn't be a migration, guardrails, versioning, and testing expectations.

Description of how you validated changes

• Unit tests for the stamping logic (semver comparison incl. rc-tag edge cases, shipped/unshipped stamping, field preservation).
• Ran the full package target: stamped dist migrations.json produced correctly against the current tag set (empty collection passes through cleanly).
• Full test suite green via pre-commit.

Issue # (if applicable)

Closes #703.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.70%. Comparing base (fe10023) to head (4b6b4a7).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/nx-plugin/src/utils/migration-versions.ts	85.71%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #928      +/-   ##
==========================================
+ Coverage   87.61%   87.70%   +0.09%     
==========================================
  Files         156      158       +2     
  Lines        5723     5830     +107     
  Branches     1363     1393      +30     
==========================================
+ Hits         5014     5113      +99     
- Misses        362      364       +2     
- Partials      347      353       +6     

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