[cavalcanticampos]

Fixes #1755.

What changes for users

agent-browser diff snapshot without --baseline now does what the help text, the docs and the MCP tool description already say: it compares the current page against the last snapshot taken on the tab. On an unchanged page it reports no changes instead of every line as an addition.

agent-browser open file:///.../snapshot_diff_probe.html
agent-browser snapshot
agent-browser diff snapshot        # before: 6 additions; now: no changes

This takes option 1 from the issue (the docs describe the intent), since the help text, README.md, docs/content/docs/diffing.mdx and the MCP description all promise it.

How it works

• DaemonState keeps last_snapshot_text, the text of the last snapshot per page session. It is bounded to 32 entries, like snapshot_revisions.
• snapshot records its tree there, for both full and delta responses.
• diff snapshot with no --baseline reads it as the baseline, and after a successful diff stores its own capture, so repeated diffs report what changed since the previous one. A failed diff (for example an unmatched --selector) changes nothing, as with the ref map today.
• With no snapshot yet on the tab, the baseline is still empty, so every line is an addition, same as before.
• --baseline <file> is unchanged.

The help text and diffing.mdx now say "on the current tab" and mention that a diff counts as a snapshot.

Verification

On Linux (WSL2), rustc 1.99:

• cargo fmt -- --check, cargo clippy -- -D warnings: clean.
• cargo test: 2051 passed, 204 ignored. Two new unit tests cover per-tab replacement and the 32-entry bound.
• New e2e e2e_diff_snapshot_without_baseline_uses_last_snapshot (headless Chrome, the snapshot_diff_probe fixture): snapshot, then a bare diff snapshot reports no change; after editing the button text, it reports exactly 1 addition and 1 removal; a third bare diff reports no change. cargo test e2e_diff -- --ignored --test-threads=1: all 3 diff e2e tests pass.
• The same new e2e on main's actions.rs fails with the bug from the issue (@@ -0,0 +1,6 @@, the whole page as additions).

Not run: the docs site suite (pnpm --filter docs ...). The docs change is one sentence in diffing.mdx.

Related: #1647 asks for -i/--interactive on diff snapshot. With this change that matters more, since a snapshot -i followed by a bare diff snapshot compares an interactive tree with a full one. I opened that as a separate PR to keep this one focused.

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