Publish committed file diffs over ACP

[haveanicedavid]

Problem

fx computes and retains an authoritative committed-file handoff containing the
canonical path, previous content, new content, and tool-call identity.

The ACP adapter currently discards that handoff:

• publishCommittedFileHandoff reports the diff as skipped.
• ACP clients receive only completion text such as
  edited README.md (5115 bytes).
• Clients therefore cannot render accurate inline diffs or file-change
  summaries.

A client cannot safely recover the missing information by parsing that prose,
watching the filesystem, or taking Git snapshots. Those approaches cannot
reliably attribute changes to a specific tool call and may include pre-existing
or concurrent edits.

ACP already defines structured diff tool-call content with path, oldText,
and newText, so no protocol extension is needed.

Change

Publish the existing committed-file handoff as an ACP tool_call_update
containing:

• The canonical absolute file path
• The exact previous content, or null for a new file
• The exact committed content
• The original tool-call ID

The subsequent completion update omits content. ACP content updates replace
the collection when present, so omitting it preserves the previously published
diff while still advancing the tool status to completed.

This uses fx's existing source of truth and adds no filesystem polling,
snapshotting, or provider-specific client behavior.

Verification

• zig build
• zig fmt --check src/
• bun test acp.test.ts -t 'ACP publishes committed file changes as authoritative diffs'

The end-to-end test drives the freshly built fx acp binary and verifies:

• The file is committed.
• The diff update contains the canonical path and exact old/new contents.
• The diff precedes the completed update.
• Completion does not overwrite the diff content.
• stderr remains empty.

Required label: type: bug.

[vercel-security-reviewer]

Security review: https://vercel.slack.com/archives/C0BSEH6GB18/p1787897765901289