feat: add tower base entry, session listing, and engine fixes (#275)

## Related Issue

None — internal maintenance pass, no external issue to link.

## Problem

Tower mode could only start from the branch the checkout happened to be
on, and there was no way
to see that a tower worker had died. Saved sessions had no listing
command, forking a large session
loaded the entire session first, and several engine-level defects were
outstanding: a session
failed to start outright when the subagent model pool in `config.toml`
was incomplete, `git status`
mis-parsed non-ASCII paths, `PYTHINKER_CODE_EXPERIMENTAL_FLAG` overrode
an explicit
`[experimental]` config entry, step retry and interrupt events were lost
on reload, and the model
was not re-reminded about subdirectory `AGENTS.md` files after context
compaction.

## What changed

**Tower mode**
- `/tower <base-branch>` picks the base branch explicitly instead of
inferring it. The recorded
  base flows through spawn, review, and the merge gate.
- The status view reports a worker that has died.

**CLI**
- New `pythinker session list` subcommand, with `--all` and `--json` for
machine-readable output.
- Forking a session no longer materializes the whole session first, so
large sessions fork in well
  under a second.
- `pythinker acp` always runs on the default agent engine and no longer
reads
`PYTHINKER_CODE_LEGACY_FLAG`; the `acp-native` subcommand and its test
are removed.

**Engine fixes**
- Sessions start when the subagent model pool in `config.toml` is
incomplete.
- `git status` parsing handles paths with non-ASCII characters (`-z`
records).
- Flag precedence is per-flag env, then `[experimental]` config, then
the master env flag, then the
  declared default.
- Step retry and interrupt wire events are persisted and fold correctly
on a cold read.
- `AGENTS.md` is re-remembered after context compaction; session start
reports the enabled flag set
  in telemetry.
- Session-index reprojection keeps the index consistent after
out-of-band session writes.
- Shorter default system prompt.

**Tooling**
- `apps/vis` moved onto the agent-core-v2 record shapes.
- The VS Code webview moved to TanStack Query.
- Transcript `docs/sdk.md` refreshed against the current `src/contract`.

Three tests that spawn real git processes had their timeouts raised from
the 5s default to 30s;
they finish in about 3s idle but exceed 5s on a loaded or slower runner.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [ ] I have linked a related issue (external PRs: the issue must have a
maintainer's `/approve`).
- [x] I have added tests that prove my feature works.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added `pythinker fork` and `pythinker session list`, including
archived-session filtering and JSON output.
* Added manual Tower base-branch selection and improved visibility into
dead workers and failures.
* Added Discussion/Expert Talk actions to take answers for editing or
build prompts directly.
* Improved streaming Fusion answers, reasoning display, and tool-call
summaries.

* **Bug Fixes**
  * Session forking is faster and remains cold until resumed.
* Improved handling of non-ASCII Git paths, incomplete model pools,
interrupted steps, and session freshness.
  * Improved Windows file reveal behavior.

* **Documentation**
  * Updated ACP, configuration, Tower, and transcript guidance.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/acp-default-engine.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+The pythinker acp subcommand no longer honors PYTHINKER_CODE_LEGACY_FLAG; it always runs on the default agent engine.

.changeset/agents-md-reminder-after-compaction.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Re-remind the model about subdirectory AGENTS.md files after context compaction.

.changeset/discussion-compact-exchange.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Compact the Discussion exchange: reasoning shows its first two sentences with a Show reasoning toggle, tool calls collapse to one summary line, and the Fusion answer streams as text instead of its raw JSON envelope. Take now loads an answer into the composer to edit, while Build from Fusion sends it as the implementation brief at once.

.changeset/discussion-scroll-and-model-pill.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Fix the transcript not scrolling up after a Discussion finishes, and show "Discussion" on the composer model pill while a Discussion is armed or running.

.changeset/fast-session-fork.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Forking a session no longer loads the whole session first, so large sessions fork in well under a second.

.changeset/flag-config-over-master-env.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+An explicit [experimental] entry in config.toml now takes precedence over PYTHINKER_CODE_EXPERIMENTAL_FLAG.

.changeset/git-status-non-ascii-paths.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Fix git status parsing for file paths that contain non-ASCII characters.

.changeset/session-list-command.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+Add the pythinker session list subcommand to list saved sessions. Run pythinker session list --all --json for machine-readable output.

.changeset/session-start-without-model-pool.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Fix sessions failing to start when the subagent model pool in config.toml is incomplete.

.changeset/simplify-system-prompt.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Shorten the default system prompt.