refactor(paths): replace eager constants with lazy getters (N4)

[NagyVikt]

Summary

Implements Theme N4 — Drop eager paths.ts constants from
docs/future/17-ROADMAP.md.

The bare exports codexDir, accountsDir, authPath, currentNamePath,
registryPath, and sessionMapPath in src/lib/config/paths.ts were
evaluated at module import time, so env-var overrides set after the first
import had no effect. Rationale lives in
docs/future/01-ARCHITECTURE.md §10.2.

All internal call sites already use the resolveX() functions; the bare
constants are now marked @deprecated and scheduled for removal in v0.2.0
so any external library consumers get a one-release migration window.

Exit criteria

• No file imports the bare constants (codexDir, accountsDir, etc.).
• The constants are marked @deprecated with a one-release removal note (v0.2.0).
• A test under src/tests/paths.test.ts proves env-var changes apply after module load.

Files changed

• src/lib/config/paths.ts — JSDoc @deprecated on six bare constants; resolvers unchanged.
• src/tests/paths.test.ts — new; 6 tests covering all five resolvers plus a regression guard.

Zero call-site conversions were required: grep confirms every importer of config/paths already pulls in resolveX() functions, not the bare names.

Verification

$ npm run build
$ npm test
# tests 91
# pass 91
# fail 0

$ grep -rn 'from .*/config/paths' src/ \
    | grep -v 'resolveCodex\|resolveAccounts\|resolveAuth\|resolveCurrentName\|resolveRegistry\|resolveSessionMap\|resolveSnapshot\|paths.ts\|paths.test'
# (only false-positive: multi-line import in account-service.ts — every symbol imported is a resolver)

Behavior change

None for end users. Pure refactor + new test. The deprecated constants still resolve to the same paths they always did.

Coordination

Sibling PRs from N1 (durability) and N3 (errors taxonomy) running in
parallel; expect a rebase if they land first. This PR deliberately did
not touch command file bodies (only resolver-import sites would
change anyway, and there were none), so the conflict surface with N3 is
empty.

Test plan

• npm run build clean
• npm test — 91/91 pass on Node 22 / Linux
• CI matrix (Ubuntu/macOS/Windows × Node 18/20/22)