feat: strategies decline with a status that names the rule that applied

[lesnik512]

Closes #97.

A strategy's no-bump answer used to be one fixed status and reason per strategy, so it could say something false:

• conventional-commits reported a valid non-bumping commit (ci: ..., docs: ...) as no_conforming_commit, "No conforming Conventional Commits type found in commit message."
• branch-prefix reported a merge commit from an unmapped branch (Merge branch 'docs/typo' into main) as no_merge_commit, "Latest commit on default branch is not a merge commit."

BumpStrategy.decide now returns Bump | Decline(status, reason), so each rule names itself, and the use case turns a Decline into the NoBump outcome. The no_bump_status / no_bump_reason class attributes are gone, and so is Bump.NONE: a decline is now the only way to say "no bump", so a strategy can't return a value the version computation can't handle. The wire bump field still reads "none".

Strategy	When	Status	Reason
conventional-commits	subject doesn't match the grammar	no_conforming_commit	Commit subject is not a Conventional Commit.
conventional-commits	valid commit, type not in minor_types/patch_types	no_bumping_type (new)	Commit type is not configured to bump.
branch-prefix	not a merge commit, patch_on_non_merge_commit off	no_merge_commit	Latest commit on default branch is not a merge commit.
branch-prefix	merge commit, no configured prefix in its subject	unmapped_branch_prefix (new)	Merge commit's source branch has no configured prefix.

Decisions (from the triage grilling on #97):

• New tokens, not just new reasons. A status token never changes meaning, but new ones may be added; keeping a token that says something false isn't what "frozen" should protect. schema_version stays "1.0". action.yml maps every non-created status to no-bump, so Action users see no change. A JSON consumer matching no_conforming_commit or no_merge_commit exactly will see the new tokens for those inputs; 0.10.0's release notes will carry an Upgrading note.
• Reasons are fixed strings. They never quote the commit type or branch prefix, which keeps reason semvertag-controlled text (see the field audit pinned in test: pin RunResult's fields so a new one forces a redaction audit #92).
• ADR-0002 is amended in place. It called the wire "a frozen machine contract of fixed status tokens and reasons", contradicting CONTEXT.md (only status tokens frozen) and the no_tags reason rewordings already shipped in feat: read v-prefixed tags and keep their prefix on the next tag #93 and feat: say how many tags were skipped when no_tags fires on a tagged repo #95. action.yml never reads reason. The ADR now says only status tokens are frozen and reason is human-facing; the rest of its decision (renderings stay split) stands.
• CONTEXT.md adds Decline and updates Strategy and Outcome to match.

Also updated: both strategy pages name each decline status; the provider guides mention unmapped_branch_prefix; the GitHub outputs table's no-bump description; and the action.yml comment now says "any other status" instead of keeping a second copy of every token.

Tests go through decide for each table row and through SemvertagUseCase.__call__ with a stub strategy returning a Decline (checked to fail with the mapping removed). Existing Bump.NONE assertions now assert the specific decline.

Verified with just lint-ci, just test-ci (500 passed, 100% coverage), just adr-check and just docs-build.