chore(process): add the feature milestones rule and the gate files

[jigarkhwar]

What

Brings the Galaxio process rules into this repository with scripts/adopt.sh from spec-kit-galaxio-bootstrap (galax-io/spec-kit-galaxio-bootstrap#22), which never replaces a file the repository has. The main addition is Feature milestones (ALWAYS) (galax-io/spec-kit-galaxio-bootstrap#20): a feature is an issue in galax-io/galaxio-sdlc-backlog; this repository carries its part under its own version milestone with Part of galax-io/galaxio-sdlc-backlog#<feature>; the backlog publishes a release when every feature of its milestone is done.

The script's report

inserted: AGENTS.md (Feature milestones)
kept: CLAUDE.md
added: scripts/check-linkage.sh
added: scripts/check-linkage_test.sh
added: .claude/hooks/linkage-guard.sh
added: .claude/hooks/linkage-guard_test.sh
added: .githooks/pre-push
added: .githooks/pre-push_test.sh

• AGENTS.md (Feature milestones): the section is inserted before ## Commits & PRs; no other line of the file changed.
• The gate files are added but not switched on: .githooks/pre-push needs git config core.hooksPath .githooks, the Claude hook needs .claude/settings.json, and no CI job runs scripts/check-linkage.sh here yet.
• cspell.json gets sdlc (the spell check flags it in galaxio-sdlc-backlog), and the Release bullet says "first em dash" because Prettier rewrites a code span with spaces at its edges.
• No .copier-answers.yml is created, so a later copier update does not read these differences as local edits.

Technical PR: no issue.

🤖 Generated with Claude Code

[jigarkhwar]

Merging on the maintainer's explicit instruction: jigarkhwar (John), 2026-10-16, in the Claude Code chat: «Мержи spec-kit-galaxio-bootstrap#22 и 13 PR chore/adopt-feature-milestones, когда все проверки зелёные; тег v0.5.0 в bootstrap поставь после мержа». Every check that ran on this pull request passed.