The terminal's issues list shows the pillar each issue serves, or Later, and `P` narrows the list to one pillar #7326

Intent: the plan reads in the terminal as it does on the map: which pillar each piece of work serves.

Done:

• [ ] Each row in the issues list (src/tui/screens/issues.ts) carries a short pillar tag (P0…P8, or later) from servedBy (src/core/pillars.ts), read once per screen, not per row.
• [ ] P cycles the filter through the repository's pillars and all; the header says which is on.
• [ ] A repository with no umbrellas shows no tag and no P key.
• [ ] A test (e.g. test/tui-issues-pillar.test.ts) makes an umbrella and an issue under it, sees the tag, and sees P hide an issue under another pillar.

Files: src/tui/screens/issues.ts, a new test.