chore: use journeys as prs qa contract

[ovitrif]

Closes #1249

Counterpart: synonymdev/bitkit-ios#743

This PR:

1. Adds scripts/journeys_index.py and the generated journeys/index.json, which lists per journey the identifiers its actions name, the source files that declare them, and the screens its actions name.
2. Adds a Journeys workflow that runs the script with --check and fails on a stale index or on an identifier that no source file declares.
3. Opens QA Notes with #### Journeys in the PR template and the /pr command, and limits #### Manual Tests to what a journey cannot express.
4. Adds a Journeys section to AGENTS.md: a PR that changes user-visible behaviour adds or updates the journeys that prove it, and updates any journey whose route it changes.

Description

The /pr command now regenerates the index, lists the journeys the branch adds or updates, and stops when a user-visible change has no journey or when a journey whose sources the branch changes no longer matches the diff.

A source file declares an identifier when it contains it as a string literal, or as a string template whose text before the first interpolation begins the identifier while the rest has no capital letters ("N$text" declares N9 but not NRemove). The script uses only the Python standard library and runs on Python 3.9 and later.

Open PRs that add or change journeys (#1231, #1225, #1217, #1170) regenerate the index after this merges. Every identifier on their current heads resolves.

Design

N/A — no UI changes.

Preview

N/A — no user-visible changes.

QA Notes

Journeys

N/A — no user-visible behaviour change.

Manual Tests

• 1. Branch that edits a journey → /pr --dry: the saved description lists that journey under #### Journeys — agent tooling, not an app flow.

Automated Checks

• python3 scripts/journeys_index.py --check passes on Python 3.9 and 3.12: 34 journeys, 91 identifiers, all declared.
• The check fails with its message when a journey names an unknown identifier, when a declaration changes without a regenerated index, when a journey is malformed XML, and when the index is missing.
• actionlint passes on journeys.yml.
• Check journeys index passed on this PR's head.
• Check journeys index on master pushes (after merge).