fix(coding-agent): copy official ~/.pi state once instead of moving it on every start

[TykanN]

Problem

legacy-senpi-dir-migration.ts runs on every start (runMigrations) and renameSyncs ~/.pi/agent, ~/.pi/mom and <cwd>/.pi into the branded layout (~/.senpi/agent, ~/.omo/agent, …). When the destination already exists it moves every entry that is missing there.

On a machine that also runs upstream pi (@earendil-works/pi-coding-agent), this empties the real pi install:

• First omo launch → ~/.pi/agent/{extensions,skills,models.json,auth.json,…} are moved to ~/.omo/agent/, leaving ~/.pi/agent empty. mtimes are preserved, so it is a rename, not a copy.
• Anything the user puts back into ~/.pi/agent is moved again on the next launch.
• Same for a project's ./.pi → ./.omo.

So pi and a branded fork cannot coexist. brand-dir-migration.ts already states the opposite rule for ~/.senpi → brand dir: "COPIED once - never moved - because the same machine may keep running the engine standalone, and a move would silently empty that install." The .pi path just never got the same treatment. From the history (1ea83112b, 9a71aae8) the move was meant for the fork's own pre-rename state in ~/.pi, not for upstream pi users.

Fix

• Official .pi sources (~/.pi/agent, ~/.pi/mom, <cwd>/.pi) are now copied once: missing entries only, nothing overwritten, and a .migrated-from-pi marker written into the destination short-circuits later starts. The original directory is left untouched.
• Sources nested inside the fork's own config dir (<config>/.pi/agent, <config>/.pi/mom, <cwd>/<config>/.pi) keep the existing move behaviour; nobody else reads those.
• The startup message now says "Copied … / The original directory is untouched", mirroring brand-dir-migration.ts.

Tests

• Updated senpi-migration.test.ts: the first case now asserts copy-once semantics (originals still exist, nested extension dir copied, marker written in all three destinations).
• New case: re-running after the marker exists does not pull newly added ~/.pi/agent files.
• Existing nested-move and symlink-sandbox cases unchanged and passing.

bunx vitest --run test/senpi-migration.test.ts   # 4 passed
bunx biome check <changed files>                  # clean
bunx tsc --noEmit                                 # exit 0

changes.md entry added per the fork contract.

Behaviour change to be aware of

Users who relied on the migration moving their ~/.pi into the branded dir will now get a copy and keep the original. That is the intended outcome of this PR; if the maintainers would rather gate the copy on something (e.g. only when the brand dir does not exist yet), happy to adjust.

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Summary by cubic

Fixes the legacy directory migration so branded agents copy official ~/.pi state into the branded layout once instead of moving it on every start, letting upstream pi and branded forks coexist on one machine.

• Official .pi directories are now copied once and left untouched; a .migrated-from-pi marker skips later starts.
• Entries already present in the destination are never overwritten.
• Pre-rename leftovers nested inside the fork's own config directory keep the existing move behavior.
• Users who relied on the move will now keep the original ~/.pi directory.

^{Written for commit 5ba4db6. Summary will update on new commits.}