Session list change counts show the whole branch divergence, not the session's changes

> **Update:** struck-through passages below are corrections to my original report — two measurements came from the wrong repository, and the suggested fix turned out to be wrong. Details, re-measured figures and further evidence in [this comment](https://github.com/microsoft/vscode/issues/330941#issuecomment-5299050850). The report itself stands.

### Summary

The change counts shown against a session in the sessions list are the whole branch's divergence from its base branch, not that session's changes. On a long-lived shared branch the number grows without bound, is identical for every session in the repository, and wildly overstates what an agent did.

### Repro

1. Work on a branch that is well ahead of the default branch (a long-lived `dev`, say).
2. Start a session there and let it edit one or two files.
3. Look at the session in the sessions list.

Observed: a count in the thousands, close to identical for every session in that repository, growing over time.

Expected: what this session changed.

### Cause

The list chip renders `changes.insertions/deletions` ([agentSessionsViewer.ts:542](https://github.com/microsoft/vscode/blob/main/src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsViewer.ts#L542)), sourced from the Branch changeset summary ([agentHostChangesetService.ts:1356](https://github.com/microsoft/vscode/blob/main/src/vs/platform/agentHost/node/agentHostChangesetService.ts#L1356)), which is `git diff` against `merge-base HEAD origin/` ([agentHostGitService.ts:474](https://github.com/microsoft/vscode/blob/main/src/vs/platform/agentHost/node/agentHostGitService.ts#L474)).

That is correct when a session owns its branch, which is the cloud/PR shape. For a local session started on an existing shared branch, the merge-base is far behind and the diff is dominated by work no session performed.

~~Measured on a real repository: a session that touched two files sat on a `dev` branch whose `git diff --shortstat $(git merge-base HEAD origin/main)` was `104 files changed, 11448 insertions(+), 1285 deletions(-)`. A second repository whose branch was level with its remote correctly showed only its uncommitted `209 insertions, 6 deletions`, which is the same code path behaving differently purely because of branch position.~~

**Corrected.** Both figures above were misreported: the first came from a different repository than the screenshot, and the second was not a second repository at all. Re-measured on the repository this report came from, on its `dev` branch:

```
$ git diff --shortstat $(git merge-base HEAD origin/main)
166 files changed, 11329 insertions(+), 634 deletions(-)

$ git ls-files --others --exclude-standard | wc -l # 17 untracked files,
17 # totalling 4741 lines

$ git diff --shortstat origin/dev # same repo, own remote
9 files changed, 209 insertions(+), 6 deletions(-) # HEAD level with origin/dev
```

`166 + 17 = 183` files and `11329 + 4741 = 16070` insertions is exactly what the chip displays (`183 files, +16070 -634`); the branch changeset includes untracked files, `--shortstat` alone does not. The `209 insertions, 6 deletions` is the *same* repository measured against its own remote branch rather than the default branch — which still makes the original point, and makes it on one repository: the identical code path reports `11329` or `209` purely according to which base it resolves.

### Suggested fix

~~The host already captures a per-session baseline checkpoint per repository (`IAgentHostCheckpointService.captureBaselineCheckpoint`, retrievable via `getBaselineCheckpoint`). Anchoring the branch diff there makes the number the session's own footprint, and leaves a branch-owning session unchanged because its baseline is the branch point.~~

~~PR follows. Happy to close it if the current definition is deliberate, in which case the counts may still be worth relabelling, since as displayed they read as the session's work.~~

**Withdrawn.** That was tried in #330942 and closed: a baseline checkpoint is a parentless snapshot commit rather than an ancestor of `HEAD`, so it cannot be used as the base of a merge-base working-tree diff — doing so compared unrelated trees and produced a worse number than the one it replaced.

No new computation is needed in any case. The `session` changeset already diffs the session's baseline checkpoint against its latest turn checkpoint via the two-ref primitive ([agentHostChangesetService.ts:1541-1566](https://github.com/microsoft/vscode/blob/main/src/vs/platform/agentHost/node/agentHostChangesetService.ts#L1541-L1566)), which is the session's own footprint. The open question is only which changeset feeds the chip — and that is a product decision, since the branch-derived chip is deliberate (#323852, pinned by a characterization test). The counts may still be worth relabelling, since as displayed they read as the session's work.

*AI disclosure: this issue was written with the assistance of AI.*

### Public patches and patcher scripts

[Public patch catalog and patcher scripts](https://github.com/RyanEwen/vscode-patches/blob/main/CATALOG.md) · [Source patch index](https://github.com/RyanEwen/vscode-patches/blob/main/SOURCE-PATCHES.md). The [public collection](https://github.com/RyanEwen/vscode-patches) includes the maintained patchers, rollback instructions, regression scripts, and historical snapshots. Build restrictions and exact installer coverage are documented there.