docs: add custom endpoint profile documentation to README

[lukeinglis]

Closes #1233 (follow-up)

Changes

• Added a "Custom Endpoints via Profiles" subsection to the Runners section of README.md
• Documents how credential profiles can reroute the claude runner to any API-compatible endpoint (e.g. a LiteLLM proxy) without a new runner class
• Includes a worked [credentials.glm] example with ANTHROPIC_BASE_URL, the [unset] sub-table for clearing conflicting vars, and a note on protected system vars
• Matches existing README tone and formatting; ~22 lines added

[RobotSail]

@lukeinglis This apparently already has conflicts with the README in main branch, can you please fix it?

[lukeinglis]

@osilkin98 Rebased onto main — resolved the README.md symlink conflict by moving the changes into docs/index.md. Ready for review.

[github-actions]

Sentrux Quality Report

Absolute

Scanning ....
[scan] git ls-files: 654 total, 642 kept, 12 dropped (ext:12, meta:0, big:0)
[build_project_map] 642 files, 106 unique dirs, 98 cache misses, 5.1ms
[resolve] 1221 resolved, 1636 unresolved (of 2857 total specs)
[resolve_imports] project_map 5.2ms, suffix_idx 1.1ms, suffix_resolve 19.4ms, total 25.8ms
[build_graphs] 642 files | maps 1.6ms, imports 25.9ms, calls+inherit 5.8ms, total 33.4ms | 1220 import, 9834 call, 11 inherit edges
sentrux check — 3 rules checked

Quality: 4492

✗ [Error] max_cc: 3 function(s) exceed max cyclomatic complexity of 30
    factory/cli/_ceo_helpers.py:_validate_ceo_flags (cc=43)
    factory/cli/_ceo_helpers.py:_execute_ceo (cc=43)
    factory/cli/run.py:cmd_run (cc=32)

✗ 1 violation(s) found

Diff (vs base branch)

Scanning ....
[scan] git ls-files: 654 total, 642 kept, 12 dropped (ext:12, meta:0, big:0)
[build_project_map] 642 files, 106 unique dirs, 98 cache misses, 5.2ms
[resolve] 1221 resolved, 1636 unresolved (of 2857 total specs)
[resolve_imports] project_map 5.3ms, suffix_idx 1.1ms, suffix_resolve 19.2ms, total 25.6ms
[build_graphs] 642 files | maps 1.6ms, imports 25.8ms, calls+inherit 6.0ms, total 33.3ms | 1220 import, 9834 call, 11 inherit edges
sentrux gate — structural regression check

Quality:      4492 -> 4492
Coupling:     0.79 → 0.79
Cycles:       4 → 4
God files:    3 → 3

Distance from Main Sequence: 0.36

✓ No degradation detected

[github-actions]

⚠️ Merge conflict detected with main

The following files conflict:

• docs/index.md

Please rebase or merge main to resolve.