…w_rescues

Continues Phase 1 of ADR-0012. With Operations now Result-returning
(previous commit), the only remaining nil-and-print pattern was
`Workflow#build_adapter`, and the only remaining typed-exception
rescue shell was `Workflow#with_workflow_rescues`. Both go.

`build_adapter` now returns `Success(adapter)` or `Failure(:unauthenticated)`.
`CLI::Fix#run` and `CLI::Fork#run` pattern-match on the Result and
print the auth-login hint themselves on the Failure branch.

`with_workflow_rescues` is removed entirely. The two verbs that used it
(Fix, Fork) handle each layer's failures explicitly:

  - build_adapter Failure(:unauthenticated) → auth-login hint, exit 1
  - bootstrap Failure(:adapter_error, msg)  → "<verb> failed: <msg>", exit 1

`CLI::Fix#execute` keeps a small targeted `rescue GemContribute::AdapterError`
on the outer def, scoped to the post-bootstrap git work that still
raises (notably `@git.checkout_branch`). The next commit (#27) folds
that into `Operations::Branch` (Result-returning) and the rescue can
go away.

`CLI::Fork#execute` doesn't need a rescue — every call site after
bootstrap is already Result-aware.

No test changes: the existing "auth-login hint" specs exercise the
verb's full flow and assert on the rendered message, so they pass
unchanged through the new Result wiring.

Closes #26.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Per ADR-0012 Phase 1, the `fix` flow becomes a four-step dry-operation
pipeline that the CLI verb (and a future TUI Command) call as a single
Result-returning service.

New operations:

  * `Operations::Branch` — wraps `git checkout -b gem-contribute/issue-<N>`.
    Returns `Success(Result(name:))` or `Failure([:adapter_error, msg])`.

  * `Operations::Announce` — posts (or skips) the "working on this"
    comment. Output-free; gating (`--no-comment`, config, viewer-self)
    is pushed in via the `allow:` argument so the operation itself is
    a pure decide/post/skip. Returns `Success(:posted | :skipped)` or
    `Failure([:announce_failed, msg])` (informational, not fatal).

  * `Operations::FixPipeline` — composes Fork → Clone → Branch → Announce
    via `Dry::Operation`. The first three are `step`s (short-circuit on
    Failure); Announce is called outside `step` so its Failure does not
    propagate as a pipeline-level failure (the fix itself succeeded).
    Adds the viewer-self gate (`fork.viewer != project.owner`) on top
    of the caller's `allow_announce:` boolean.

CLI/library changes:

  * `CLI::Fix` swaps its `fork:` (CLI::Fork instance) dependency for
    `pipeline:` (FixPipeline). The verb is now a thin Result-pattern-
    matching shell: build the adapter, compute `allow_announce`, call
    the pipeline, render the summary and announce outcome. The
    targeted `rescue GemContribute::AdapterError` introduced in #26
    is gone — the branch step now returns Failure too.

  * `CLI::IssueAnnouncer` keeps `fetch_claim_index` (used by `scan` and
    `issues` to flag claimed issues) and exposes `MARKER` as an alias
    for `Operations::Announce::WORKING_MARKER` so the search query
    stays in sync with the marker the operation posts.

Specs: three new operation specs cover Branch, Announce, FixPipeline
(11 new examples). `cli/fix_spec.rb` swaps `fork: fork_cli` stubbing
for `pipeline: pipeline` stubbing and adds explicit coverage for the
allow_announce-gating cases (--no-comment, config off, per-repo
override) and the announce-outcome rendering (Success(:posted),
Success(:skipped), Failure([:announce_failed, ...])).

Closes #27.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

The two `# rubocop:disable Metrics/ParameterLists` comments were
load-bearing only because each verb's initializer juggled 9-10 keyword
args plus the `value || Default.new` fallback dance. `dry-initializer`'s
`option :name, default: -> { ... }` syntax expresses the same thing
declaratively, evaluates defaults in declaration order (so a later
option's default can reference an earlier option), and produces the
same external interface.

Notable trickle-through:
  * `option :post_clone_hooks` references `stdout` / `stderr`
  * `option :pipeline` (Fix) and `option :clone_op` (Fork) reference `git`

Both work because of the in-declaration-order default semantics.

Both rubocop disable directives are gone. No behaviour change; existing
specs pass without modification (the keyword-arg constructor surface is
identical).

Closes #28.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>