fix(store): allow shared leases on read-only indexes

[sunerpy]

Summary

• open the permanent index lock read-only for shared leases
• retain read/write access for exclusive lifecycle operations
• add a regression test that proves a write-capable handle is denied while shared acquisition succeeds

Verification

• make ci
• cargo build --release -p codegraph-rs
• release-binary smoke test: status and strict search succeed with .codegraph/index.lock mode 0444
• index content hash remains unchanged across the read-only smoke test

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/codegraph-store/src/index_lease.rs	50.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   95.36%   95.37%           
=======================================
  Files         142      142           
  Lines       80049    80050    +1     
=======================================
+ Hits        76341    76345    +4     
+ Misses       3708     3705    -3     

Files with missing lines	Coverage Δ
crates/codegraph-store/src/index_lease.rs	88.59% <50.00%> (+0.02%)	⬆️

... and 1 file with indirect coverage changes

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