fix(mcp): one release point for handle_index_repository's argument strings

main fails the memory-core linter:

    src/mcp/mcp.c: grew by 3 (814 -> 817)

PR #1723 (merged as 92abefa3) added a new early-return path to
handle_index_repository and, following the function's convention, freed
its three argument strings there with raw free(). Its CI lint was green
on 2026-09-09. On 2026-09-19 the memwaste merge cut this file's raw sites
and ratcheted the baseline down to 814, so by the time #1723 landed its
green was measured against a rule that no longer existed. The linter is
right and the merge-result check that let it through was incomplete: it
built and ran the suites and did not run the linter. It does now.

The fix is not cbm_free. The three strings come from
cbm_mcp_get_string_arg and resolved_repo_path_from_project_arg and are
raw heap memory, released raw by every other path in the function;
routing three of them through the core would free raw memory through the
accounting layer. The actual defect is that the function had 41 raw
free() calls of the same three arguments across a dozen early returns,
and #1723 added one more block in the same shape.

One helper, index_args_free(repo_path, mode_str, name_override), now
replaces twelve of those blocks. free(NULL) is a no-op, so the one path
where repo_path is still NULL passes NULL. On the cross-repo branch
mode_str used to be freed before handle_cross_repo_mode and the other
two after it; mode_str is neither passed to nor read by that call, so
releasing all three after it is not observable. Five sites on the
normal-completion path are left as they were: they free the strings at
different points as ownership is handed to the pipeline, and collapsing
them would change that order for no gain.

Raw sites in this file: 817 -> 785. The baseline follows the improvement,
814 -> 785, as the linter asks; it can only tighten.

Build: clean, -Werror, ASan/UBSan. mcp cli daemon_application:
692 passed, 4 skipped, 0 failed -- index_repository_honors_allowed_root,
which runs through a replaced early return, included.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>