docs: ADR index and template; bring stale decision records up to date

[WilliamKarolDiCioccio]

Summary

This PR makes the ADRs we already have a reliable base for the ones still to come. It changes docs only.

• docs/adr/README.md (new)
  • When an ADR is warranted: it binds forks, the backend, deployers or IdP operators; it sets a contributor-wide rule; it has security impact; or the topic has been rewritten or argued about more than once.
  • An index of ADR-001 to ADR-003. It also points out that the "Riverpod is DI only" rule lives in ADR-001 §5.3.
  • The status vocabulary: Proposed, Accepted, Superseded and Deprecated. An ADR moves to Accepted in the PR that implements it.
  • Amend or supersede: a narrow change gets a dated Amended by note, and a reversal gets a new ADR.
  • A template that follows the shape the existing ADRs already use.
• ADR-003
  • Status changes from Proposed to Accepted. It shipped in refactor: reify the Flavor as a declaration object (ADR-003) #428 and refactor: build out the Flavor declaration object (ADR-003) #430.
  • The undated "Since superseded" blockquote becomes a dated amendment by feat: let a whitelabel flavor add a screen reachable without a session #536, which is also listed in the header.
  • §4 no longer says runSoliplexShell(ShellConfig) is unchanged.
• docs/plans/0001-app-shell/proposal.md: a "partly superseded" note. ModuleContribution / "no base class" was replaced by AppModule (feat(core): AppModule lifecycle replaces ModuleContribution #172), and flavor functions by Flavor (ADR-003).
• docs/developer-setup.md: removes the GIT_LFS_SKIP_SMUDGE step. ag_ui has been a hosted pub.dev package since d8dac7e, and no workflow sets the variable any more, so the doc's "CI sets this automatically" was also wrong.
• AGENTS.md: "Key Documentation" now lists docs/adr/, the flavor authoring guide, the diagnostics risk register and the schema-snapshot procedure. The shell proposal is labelled historical.

Not in this PR

• Splitting ADR-001, which covers about six separate decisions. Its axes are already numbered, so splitting can wait until one of them needs to change.
• Any new ADR. More are planned. The first ones will cover the module contract and layering (ahead of refactor: enforce module layering (core → modules, lobby ↔ room cycle) #607) and the timeline/runs model (ahead of Split _RoomScreenState: one State class owns six unrelated concerns #601).

Test plan

• markdownlint-cli2 passes on every touched file.
• Relative links from the proposal to ADR-001 and ADR-003 resolve.
• pubspec.lock resolves ag_ui as source: hosted, and no pubspec has a git: dependency.

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