crates/minimal: cmd_dump accepts --arch to override target - #146

* crates/minimal: cmd_dump accepts --arch to override target

Closes #125. Previously `minimal dump --packages` always evaluated
Nickel against the host system's target, so packages whose
`build_deps` use `match { arch = 'Amd64 } => ..., { arch = 'Arm64 } =>
... } target` only surfaced one arm's data in the dump output — the
other arm was erased at evaluation time with no breadcrumb.

`mctx::graph_from_all_packages_with_target(target)` already exists;
cmd_dump simply wasn't plumbed to it. This commit adds a clap arg:

    minimal dump --packages --arch amd64
    minimal dump --packages --arch arm64

Parsing follows `cmd_materialize`'s existing `--arch` convention
(arm64/aarch64 → Arm64, amd64/x86_64 → Amd64). OS is always Linux;
dump consumers want per-arch evaluation of the same pkgs repo, not
cross-OS targets. When `--arch` is absent, defaults to `Target::host()`
(preserves current behavior).

Unblocks the downstream consumers listed in the issue — pkgmgr's
updater can now see per-arch source data for arch-matched packages
(deno, go, gcloud, etc.) by running the dump once per arch, and
supply-chain's scanner gets the same visibility.

Tests: resolve_target helper has unit tests covering the default path,
both arch aliases (amd64/x86_64, arm64/aarch64), and the error case
for unsupported values.

* cmd_dump, cmd_materialize: centralize arch parsing in common::Arch

Addresses review feedback: the str-to-Arch parsing (arm64/aarch64,
amd64/x86_64) was open-coded separately in cmd_dump and cmd_materialize
— moving it to a shared `impl FromStr for Arch` in common means:

- Single source of truth for which aliases we accept. uname -m output
  works verbatim on both macOS (arm64/x86_64) and Linux (aarch64/amd64).
- New consumers can use `"amd64".parse::<Arch>()` instead of
  re-implementing the match.
- Error formatting ("unsupported architecture: X. Use 'amd64' or
  'arm64'.") lives in one place alongside the parse logic.

Both cmd_dump::resolve_target and cmd_materialize's arch resolution
now delegate to this via `str.parse::<Arch>()`. cmd_materialize loses
the anyhow import shadow (only uses anyhow! from below now). Tests
for the parser move to common/target.rs; cmd_dump tests retain their
integration coverage (resolve_target defaults, Linux-OS wrapping,
error propagation).

* common, decode: FromStr for Target, replace ad-hoc loop in builds.rs

Addresses the second review point on #146: decode/src/builds.rs was
looping over Target::all() to find a target by its as_ref() string
("amd64/linux", etc.). Now that Arch has FromStr, Target can too —
consumers outside cmd_dump should use the shared impl instead.

Adds:
- FromStr for OS (linux/macos) with OsParseError
- FromStr for Target, delegates arch/os segment parsing to the
  component FromStrs. Round-trips with Target::as_ref().
- TargetParseError surfaces which segment failed (shape / arch / os)
  so users get a specific message.

decode/src/builds.rs's "target" field parser now does
`target_str.parse::<Target>()` and folds into the existing
Error::InvalidTarget variant on parse failure. No behavior change for
valid inputs.

Also fixes CI fmt-check failure: rustfmt on CI formats the
cmd_materialize arch-resolver chain differently than my initial
layout. Reformatted to match.

Tests: 7 new in target.rs — OS both variants + unknown rejection,
Target round-trip over every Target::all() value, missing-separator
error, arch error propagation, OS error propagation.

)

Removes the agentic-workflow artifacts that landed here via the old sync /
caller-wrapper install model (gominimal/min-aw PR #146 retired that path).
This repo will be re-bootstrapped via `gh aw add gominimal/min-aw/<chore>@<release-tag>`
once this PR merges — the gh-aw [sharing-workflows](https://github.github.com/gh-aw/guides/organization-practices/sharing-workflows/)
pattern.

Removed (agentic only):

- `.github/workflows/sync-from-source-of-truth.yml` — retired sync workflow.
- `.github/workflows/worker-fix.yml` + `worker-iterate.yml` — retired caller
  wrappers (no compiled `.lock.yml` was ever committed on this repo).
- `AGENTS.md`, `.github/agents/`, `.github/labels.yml`,
  `.github/copilot-instructions.md` — contract template; will be reinstalled
  by `quick-setup.sh` at re-bootstrap.
- `.github/ISSUE_TEMPLATE/{bug,chore,feature,feedback}.md` — installed by
  the min-aw contract template. The pre-existing `bug_report.md` and
  `feature_request.md` are NOT touched.

Kept (native CI + pre-existing):

- `.github/workflows/ci.yml` — Minimal CI gate.
- `.github/workflows/promote.yml` — release promotion workflow.
- `.github/ISSUE_TEMPLATE/bug_report.md` + `feature_request.md` — pre-existing
  GitHub-default issue templates.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>