fix: mark local runner execs as placement-resolved (#8115)

[chubes4]

Summary

• Fixes the recursive process-spawn storm when inspecting/checking the wp-codebox component (bug: WP Codebox component inspection recursively spawns runtime checks #8115), where homeboy component show / extension show and check-wp-codebox-runtime-core.mjs spawn each other unbounded and saturate the host.
• Root cause: prepare_runner_process only stamped the dispatch-only placement markers (HOMEBOY_RUNNER_HOSTED_EXEC, HOMEBOY_RUNNER_PLACEMENT_RESOLVED, HOMEBOY_RUNNER_ID) for non-local runners (runner.kind != RunnerKind::Local). Unify resource-aware execution placement #7998 introduced the placement-resolved marker + explicit --placement but left the local gap.
• Consequence: nested Homeboy subprocesses spawned during a local exec (extension parity preflight extension show, extension materialization, and the WP Codebox ready_check) never saw that placement was already resolved. Carrying an explicit --placement, they re-entered route_after_parse and re-dispatched — recursively spawning component/extension inspection plus the ready_check.

Fix

• Stamp the three placement markers in the local branch of prepare_runner_process, matching the already-correct daemon-local path (which documents the same "keep out of argv so nested Homeboy commands do not route themselves a second time" intent). Nested commands now detect the resolved placement via is_managed_runner_placement_context() and short-circuit routing instead of recursing.

Investigation notes

• Confirmed the extension side is clean: check-wp-codebox-runtime-core.mjs is a pure import() and the only homeboy component show call in homeboy-extensions (validation-dependencies.sh) is cycle-guarded and only reached from lint/audit/test/bench — never the ready_check or component show.
• Bounded local repro (forced-failing ready_check) showed the plain runner exec path already fails fast without looping; the loop requires the explicit---placement + missing-local-marker combination introduced by Unify resource-aware execution placement #7998.

Testing

• cargo fmt --check clean.
• Updated local_runner_prep_marks_placement_as_resolved (formerly local_runner_prep_does_not_mark_commands_as_runner_hosted, which encoded the buggy behavior) to assert the markers are now present. Heavier cargo build/cargo test deferred to CI per repo build policy.

Closes #8115