feat(mcpsrv): spec_comment closes the agent half of the review loop (spec-by6.3.4)

An agent can now read the review threads on a proposal and reply to them. It
cannot open a thread or resolve one, and that is enforced by the type rather
than by the handler remembering: spec_comment is written against a narrow
Commenter interface naming only Threads, ReplyTo, GetProposal and ProposalDiff,
so service.CommentOn and service.ResolveThread are unreachable from it however
service/ later grows. An unresolved thread suppresses policy auto-merge, so an
agent able to open or resolve one would hold the gate that exists to hold its
own output back. Writer is now the union of Proposer and Commenter, one narrow
interface per write tool, and each handler takes only its own half.

Every listed thread carries its anchor state, resolved against the branch as it
stands now rather than as it stood when the comment was written — often the
same agent has revised it since. An agent told only "fix this paragraph", with
no signal that the critique no longer describes any block, edits the wrong
thing. The tool description spells out what anchored/edited/outdated mean and
says plainly that replying does not close a thread, so an agent answers the
critique and pushes a revision instead of replying and waiting.

Replying requires the proposal as well as the thread. A thread id is a global
integer and service.ReplyTo needs nothing else, so a mistyped id would post a
reply onto a stranger's proposal, out of sight of the agent that wrote it; the
membership check reuses the threads already read for the ACL and costs nothing.

Reads stay on the uniform owner+agents gate rather than being narrowed to the
proposal an agent authored. Agent identity is self-declared in X-Agent headers
and all agents share one token, so an authorship check would constrain a string
the caller picks — stricter on paper than the read plane it sits in, and
enforcing nothing.

No wiring change was needed outside this package: main.go already passes
Write: svc, and *service.Service satisfies the widened Writer.

spec-by6.3.4

feat: service.Archive — one accessor, one tree walk, one link graph

web/ and mcpsrv/ both needed a *doc.Archive and each invented its own way
to get one. web/ handed sp.Repo to doc.Scan — reaching past service/ into
gitx, which the layering rule forbids — and then listed the documents a
second time for their bodies, two tree walks per page view. mcpsrv/
converted []service.Document back into []gitx.Document, rebuilding hashes
service/ had already stringified. Two workarounds around one missing
accessor is how three agent-facing surfaces stop being identical.

Service.Archive resolves the revision, walks the tree once, and returns
the archive with the bodies. ArchiveFrom is the same construction over a
Document set a caller already holds — the one place a hex object name is
converted back to a hash, malformed ids refused rather than zeroed.

doc.Page.Links and Page.WordCount were documented as "filled in by a
render pass" and nothing filled them, so Archive.Backlinks always
returned empty; web/ worked around that by re-rendering every document of
the space on each page view. doc.Archive.LinkPass now owns the pass and
the accessor runs it, so the link graph exists wherever the archive does.

web/ and mcpsrv/ no longer import gitx or go-git, in tests either.
doc.DirOf replaces the three copies of the same fromDir helper.

feat(mcpsrv): MCP read tools, with Host validation replacing the SDK guard

Adds the Phase 2 read tools (spec_search, spec_read, spec_list) over the
service layer.

Two security fixes came out of building them.

The read plane could serve proposal content. gitx resolves ref names, and
service.resolveRev passed any string through, so rev=proposals/42 made the
READ plane hand back unreviewed text — which would then flow into agent
context as though approved, the single failure this service exists to
prevent. ValidateReadRev now admits only the approved-head sentinel or a
full 40-character object name, at the layer all three surfaces share.
Abbreviations are refused too: one that is unique today can become
ambiguous later, so a pinned revision would silently stop meaning one
thing.

The MCP SDK's DNS-rebinding guard rejects a loopback listener whose Host
is not loopback, which is exactly nginx forwarding to 127.0.0.1 — it would
403 only in production, passing every local test. The SDK offers no
allowlist, so the guard is disabled and replaced by a stricter check: Host
must equal the configured origin, or a loopback name for development. A
rebinding attack carries the attacker's name in Host and fails it. An
unusable origin logs loudly rather than quietly unguarding the endpoint.