feat(wta): show all session origins in session management picker

[DDKinger]

Summary

The /sessions session-management picker shipped in ""MVP"" mode that surfaced only Class B (shell-pane) sessions — the user manually ran copilot / claude / gemini in a regular shell — and hid Class A (agent-pane) sessions that WTA spawned on behalf of an Intelligent Terminal agent pane. Both classes already route correctly through session_mgmt::decide_enter_action, so there's no reason to keep agent-pane sessions out of the picker.

This PR makes the picker show all session origins.

Changes

• app.rs — Replace the MVP_SESSIONS_ORIGIN_FILTER (ShellOnly) constant and the resolve_sessions_origin_filter() env-override resolver with DEFAULT_SESSIONS_ORIGIN_FILTER = OriginFilter::All; the field is initialized to it directly. Remove the obsolete WTA_SESSIONS_SHOW_AGENT_PANE env-override and its test.
• agent_sessions.rs / main.rs / cli_tests.rs / ui/agents_view.rs / tools/wta/AGENTS.md — Update doc comments to reflect the new default.

The OriginFilter axis, the App::sessions_origin_filter field, and wta sessions list --origin shell|agent-pane|all slicing are all preserved — only the picker's default changes. No Enter/Shift+Enter dispatch logic needed to change.

Verification

• cargo build — succeeds
• cargo test — 711 passed, 0 failed
• No remaining references to the removed symbols anywhere in the tree

[Copilot]

Pull request overview

This PR updates the WTA /sessions session-management picker to show sessions from all origins by default (both shell-pane “Class B” sessions and agent-pane “Class A” sessions), aligning the picker’s default behavior with the existing routing support in session_mgmt::decide_enter_action.

Changes:

• Switch the /sessions picker default origin filter from “shell only” to OriginFilter::All, removing the prior MVP-only gate and its WTA_SESSIONS_SHOW_AGENT_PANE env override.
• Update CLI/docs/comments to reflect that all is the default and that --origin shell|agent-pane remains available for slicing.
• Remove the env-override test and adjust session-origin filter tests to explicitly set filters where needed.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/wta/src/app.rs	Changes the picker’s default origin filter to All and removes the env-based override plumbing and test coverage for it.
tools/wta/src/agent_sessions.rs	Updates origin-filter documentation to match the new picker default and clarify CLI slicing behavior.
tools/wta/src/ui/agents_view.rs	Updates UI-layer comments to reference the new default and constant naming.
tools/wta/src/main.rs	Updates wta sessions list help text/comments to reflect that /sessions now defaults to all.
tools/wta/src/cli_tests.rs	Updates CLI parsing test commentary to align with the picker default and origin slicing story.
tools/wta/AGENTS.md	Updates design/behavior documentation to state the picker now surfaces all session origins and removes the obsolete env override guidance.