One worker system: retire the Keychain product path + single-worker spawn model (remaining scope from PR #5632)

## Combined Core execution: C07

[Core execution plan]() owns order and scope. This issue contributes to C07; the linked plan owns the complete packet and any outstanding candidate acceptance. Record this issue's path claims and implementation evidence here.

**Issue mapping:** [SHA-6315 / GitHub #5718]()
includes the one-worker/profile-inheritance migration. Credit the already
landed account-session Keychain prompt fix; reconcile every remaining item:
parent-profile/permission inheritance, non-git and read-only parallel workers
without forced worktrees, `/mcp connect github` add/login chaining, and
autosave `title_source` provenance. Coordinate credential-backend retirement
with C04's secrets owner and explicit migration disposition; do not silently
grant full permissions or delete a credential backend. The old PR is a
rebase-and-repair source, not a merge-ready implementation.

[SHA-6282 / GitHub #5575]() requires one derived
role/profile/dispatch description contract shared with C05, without
contradictory worker promises or elevated child permissions.
[SHA-6252 / GitHub #5529]() adds worker budget soft
landing, preserved workspace/checkpoint/report, exact-route credential dispatch
and truthful foreground/background shell results. Reconcile current source and
qualify those paths in C14; the older proposal grants no blanket authority to
commit work or switch providers on failure.

**Owner: execution owner. Dependencies: C04–C06.** Inventory and preserve
child budgets, route pins, permission/tool activation, steering, parking,
checkpoints, nesting and terminal fan-in. Adapt child work to the existing
engine execution implementation, then delete `run_subagent`'s duplicated
model-request/tool-result cycle. Agent/task tools call runtime capabilities;
they do not become a second runtime. Keep this behavioral convergence separate
from C08's mechanical move.

**Completion evidence required:** actual parent/child call paths share execution, with local
mock-provider turns, tool approval/denial, streaming, cancel/steer, nested work,
parking/resume and exact-once terminal fan-in passing. The existing guard that
counts functions named `run_turn` cannot prove this; prove the call path and
behavior. Preserve C01's worker ownership and history fix.

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## Original issue and contributor history

Tracking the remaining scope of PR Hmbown/CodeWhale#5632 (being closed — see the receipt there). The urgent slice already landed as Hmbown/CodeWhale#5662 (account sessions stop popping Keychain). What remains, verified against current main:

1. **Full OS-keyring retirement**: main still carries the complete `KeyringStore`/`DefaultKeyringStore` backend in `crates/secrets/src/lib.rs`. The design: `CODEWHALE_SECRET_BACKEND=system|keyring` becomes a no-op, secrets live in `~/.codewhale/secrets/` mode 0600.
2. **Single-worker spawn model**: subagent spawns inherit the parent profile, roles become labels, the permission-preset matrix goes away (`tools/subagent/*`, `simple_worker.rs`, `worker_profile.rs`).
3. `/mcp connect github` **chaining** add+login.
4. Two Devin findings that must be addressed in any revival (from the PR review):
* BUG `tools/subagent/mod.rs:9387-9396`: parallel spawns forced into git worktrees (`should_isolate_worktree(false, parallel)`) — fails in non-git workspaces and isolates read-only fan-out needlessly.
* Root spawns getting full permissions in every mode (`allow_shell`, `permissions=full()`, `shell=Full` overriding the parent profile) — needs an explicit intent decision.
5. `title_source` provenance dropped on autosave (session_manager.rs, latent).

The conflict surface is large (17 files also touched on main since the merge-base), so this is a deliberate rebase-and-repair effort, not a merge. Closes the CLI side of cwc#154 when done.

Start with the linked C07 core execution plan, then inspect crates/secrets/src/lib.rs, tools/subagent/mod.rs, tools/subagent/*, simple_worker.rs, worker_profile.rs, and session_manager.rs. Trace the current parent/child call paths and run the listed local mock-provider, approval, streaming, cancel/steer, nesting, parking/resume, and terminal fan-in checks. Done means the remaining migrations and shared execution behavior are reconciled without violating the stated C04–C06 dependencies or C01 ownership fixes.

Written by the indexing model from the issue text.