Inline code review comments across forges

## Motivation

git-spice is uniquely positioned to sit at the center of the code review
workflow. It is the only genuinely open-source option for *stacked* code
review, and it already provides a uniform, forge-agnostic interface for
submitting and managing change requests across GitHub, GitLab, and
Bitbucket. That makes it the natural place to offer a uniform code
**review and merge** experience — not just submission.

Inline (diff-line) review comments are the missing piece of that
experience today. Reviewing a stack means leaving the tool to comment in
the browser, per-forge, with no local or batchable workflow. Adding
first-class comment support lets git-spice own the review loop the same
way it already owns the submission loop.

Just as importantly, a uniform comment API lets *other* tools participate
in the commenting workflow through git-spice rather than reimplementing
per-forge comment plumbing themselves. The VS Code extension is the
motivating example: it can drive review comments across any supported
forge by going through git-spice, instead of speaking each forge's
review API directly.

## Proposal

Add infrastructure for posting, listing, resolving, and editing inline
review comments on pull/merge requests across GitHub, GitLab,
Bitbucket, and ShamHub, plus CLI commands to drive it. Comments can be
staged locally and submitted as a single batched review, mirroring the
stack-oriented model git-spice already uses for submission.

Key pieces:

- A `diffmap` package translating working-tree `file:line` references
into forge inline-comment coordinates (path, line, side) by parsing
unified diff output.
- New optional forge capabilities (`WithInlineComments`,
`WithThreadResolution`, `WithCommentEdit`) with per-forge
implementations (GitHub GraphQL, GitLab discussions, Bitbucket REST,
ShamHub).
- Local staged-comment storage in the spice state store so review
feedback can be drafted offline and submitted as one review.
- CLI commands for managing and submitting review comments, with
stale/outdated detection from diff state, `pr`/`file` scope variants,
and a documented `list --json` shape for external tooling (such as the
VS Code extension).

## Tracked by

- #1162 — inline comment support across forges (infrastructure)
- #1163 — CLI commands for managing review comments
- #1164 — inline review comment commands and forge support
- #1222 — compute outdated/stale from diff state
- #1223 — `pr` and `file` scope variants
- #1224 — extend `list --json` shape per extension contract