feat: tower manual base entry, session list, and engine cleanups

Tower mode accepts an explicit base branch via /tower <base-branch> and
reports worker deaths in the status view. Adds the `pythinker session list`
subcommand with --all/--json, and a fast session fork path that no longer
materializes the whole session.

Engine and CLI fixes: sessions start when the subagent model pool is
incomplete, git status parses non-ASCII paths, an explicit [experimental]
config entry outranks the master env flag, step retry and interrupt events
persist across reloads, and AGENTS.md is re-remembered after compaction.
The acp subcommand always runs on the default engine; the acp-native
subcommand is removed.

Also: shorter default system prompt, session-index reprojection, vis moved
onto agent-core-v2 record shapes, and the VS Code webview moved to
TanStack Query.

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/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.

.changeset/step-interrupt-events-persisted.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Interrupted steps keep their reason after a session is reloaded.

.changeset/tower-mode-manual-entry.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+Enter tower mode with /tower <base-branch> to pick the base branch yourself, and see when a tower worker dies in the status view.