feat(core): support isolated mesh data directories

[ovidius72]

Why

An application embedding agent-comms needs to keep its keys and grants separate from an existing installation. slot.dir moves device-slot files, but the bridge factory still loads the shared account and machine keys from the default directory.

Changes

• Add IdentitySlot.dataDir for all persistent mesh data and AGENT_COMMS_DATA_DIR as the default for shipped bridges and account commands.
• Resolve the directory in one place and reuse the existing identity, trust, naming and ledger functions.
• Keep ~/.agent-comms when the environment setting is absent or blank. Preserve the existing scope of slot.dir.
• Use the same slot for identity loading, fronted identities and lock release. No per-bridge changes or data migration.

Verification

• 13 new tests cover directory precedence, all four bridge factories, unchanged existing data, separate identities, account export/import, a real two-peer room message, restart persistence and grant revocation.
• Build, lint, typecheck and the full test suite: 1,870 tests across 211 files passed.
• 7 delivery tests, the multi-process smoke test and 53 browser tests.

Checks on fork PRs

My previous fork PR #335 had no workflow runs and was recreated as #394 so checks could run. Maintainer edits are enabled on this PR; please approve the fork workflows if approval is needed.