Add experimental Go module proxy support (#358)

* feat: add experimental Go module support via pmg go

Adds Go modules as a proxy-guarded ecosystem, opt-in only: the command
runs solely when invoked explicitly as `pmg go ...` and is deliberately
excluded from setup aliases and PATH shims so existing users are
unaffected.

- packagemanager: goPackageManager with fail-safe command classification
  (vet/fix excluded from non-download since they can fetch on a cold
  cache) and pinned-version extraction where only canonical semver
  counts as explicit.
- GOPROXY normalization (fail-closed): effective GOPROXY read via
  `go env` (honors go env -w), rebuilt comma-joined with `direct`
  dropped so a 403 block is terminal and nothing silently falls back to
  unanalyzed VCS fetches. GOPRIVATE/GONOPROXY surface a warning;
  GOINSECURE is cleared. Contributed to the proxy flow through a new
  ProxyRoutingProvider hook (extra child env + dynamic MITM hosts).
- Go interceptor with dynamic host matching from the user's effective
  GOPROXY via InterceptorContext.GoProxyHosts. Malware analysis runs on
  .zip only (the sole endpoint that delivers code); .info/.mod/@latest/
  list pass through; /sumdb/ traffic and sum.golang.org are never
  touched so checksum-db verification stays intact; golang.org/toolchain
  is allowed on Go's own checksum verification.
- Dependency cooldown: publish time captured from .info responses
  (body unmodified), in-window .zip blocked with 403; fails open for
  cooldown only when the publish time was never observed.
- Cert gate: on macOS/Windows `pmg go` fails fast with actionable
  guidance unless the persisted PMG CA is OS-trusted (Go ignores
  SSL_CERT_FILE there); Linux works via the injected bundle.
- proxye2e: GOPROXY-protocol mock registry, Go driver and 10 hermetic
  cases (allow/block/confirm, case-escaped paths, cooldown block and
  fail-open, toolchain, sumdb passthrough).

Verified end-to-end on Linux: `pmg go get github.com/google/uuid@v1.6.0`
MITMs proxy.golang.org, analyzes the decoded module at the .zip fetch,
and go.sum verification succeeds through the tunneled checksum db.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014xuhBeTVpfU4SdqVaarvuK

* fix(go): address review findings on experimental Go support

- Drop fmt/clean from NonDownloadCommands: both load packages via go
  list and can download modules on a cold cache, which would bypass the
  proxy under install_only.
- Support GOPROXY entries with a base path (e.g. corp Athens/JFrog at
  https://corp/goproxy): the interceptor now receives host -> base URL
  and strips the path prefix before parsing module URLs, so verdicts
  and cooldown key on the real module path.
- Default unschemed GOPROXY entries to https, matching go's own
  behavior, so corp mirrors configured as bare hosts are intercepted
  instead of silently unanalyzed.
- Memoize the final verdict per module zip: go re-requests a failed
  zip during go get's load phase, which double-recorded stats (the
  report showed the same blocked module twice) and would have
  re-prompted on Confirm verdicts.
- Fetch .info out-of-band on a cooldown cache miss: go serves .info
  from its local module cache on any machine that used go before PMG,
  which silently disabled cooldown. Failure of the side-fetch still
  fails open for cooldown only.
- Move the noop package resolver into packagemanager.

Verified live: cold-cache cooldown block now records once; warm-cache
rerun is blocked via the side-fetch instead of failing open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014xuhBeTVpfU4SdqVaarvuK

* docs: collapse Go proxy-mode details by default

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014xuhBeTVpfU4SdqVaarvuK

---------

Co-authored-by: Claude <noreply@anthropic.com>

docs/proxy-mode.md

@@ -69,6 +69,37 @@ Legacy variables `PMG_PROXY_MODE` and `PMG_PROXY_INSTALL_ONLY` (for the old flat
 | `uv`            | ✅      |
 | `uvx`           | ✅      |
 | `poetry`        | ✅      |
+| `go`            | 🧪 experimental |
+
+### Go (experimental)
+
+`pmg go` guards Go module downloads through the same proxy flow. It is
+experimental and opt-in: it only runs when invoked explicitly as `pmg go ...`
+and is deliberately excluded from `pmg setup` shell aliases and PATH shims.
+
+<details>
+<summary>How it differs from npm/PyPI</summary>
+
+- The module proxy host comes from the effective `GOPROXY` (including
+  `go env -w` values), not a fixed registry. PMG intercepts whatever HTTPS
+  proxies are configured and rewrites the child's `GOPROXY` to a fail-closed,
+  comma-joined list: `direct` is removed (a module PMG cannot inspect fails
+  instead of silently bypassing analysis) and pipe separators collapse to
+  comma so a block is terminal.
+- Malware analysis and dependency cooldown run on the `.zip` source download —
+  the only GOPROXY endpoint that delivers code. `.info`/`.mod` metadata passes
+  through (cooldown reads the publish time from `.info` without modifying it).
+- `sum.golang.org` is never MITM'd and `/sumdb/` requests pass through
+  unmodified, so Go's checksum-database verification stays fully intact.
+  Toolchain downloads (`golang.org/toolchain`) are allowed on Go's own
+  checksum verification.
+- On macOS and Windows, Go only trusts the OS trust store, so
+  `pmg setup cert install` is required first; `pmg go` fails fast with
+  instructions if the PMG CA is not trusted. Linux works out of the box.
+- Modules matching `GOPRIVATE`/`GONOPROXY` are fetched directly from their
+  VCS host and are not analyzed; PMG warns when these are set.
+
+</details>
 
 ## References