docs(fix): correct nav link case + relative-path level

Gitea (and any case-sensitive filesystem) was rendering the [Top]
nav links in /docs as broken because of two bugs:

1. Case-sensitivity: 22 links used '../README.md' (all-uppercase)
   but the actual file is 'docs/Readme.md' (capital R, lowercase
   rest). 21 guide_*.md nav bars were affected, plus 1 internal
   cross-link in Readme.md itself. Works on Windows (case-
   insensitive) but broken on Linux/Gitea.

   Fix: 22 occurrences across 22 files changed
   '../README.md' -> '../Readme.md'

2. Wrong relative-path level: 16 links used '../../conductor/...'
   from 'docs/guide_*.md' to reach 'conductor/'. This goes up 2
   levels to 'projects/', which doesn't exist. The correct path
   from 'docs/guide_*.md' to 'conductor/' is 1 level up
   ('../conductor/...'). 12 unique patterns across 10 files
   affected.

   Fix: 16 occurrences across 10 files changed
   '../../conductor/' -> '../conductor/'

3. Bonus: 1 planned-guide link in guide_context_curation.md
   referenced a never-written 'guide_context_presets.md'. The
   ContextPreset schema is now fully covered in the new
   'guide_context_aggregation.md' (per the 2026-06-08 docs
   refresh). Fix: link target updated.

No content was changed, only link paths. 24 files, 37 link
replacements, 37 deletions.

Verification:
- All .md links in docs/ now resolve to existing files
  (validated by path-resolution check from each file's directory)
- The 3 new guides from the previous docs refresh commit
  (guide_discussions.md, guide_state_lifecycle.md,
  guide_context_aggregation.md) had the case bug inherited from
  guide_architecture.md's existing nav pattern; their top-of-file
  nav bars are now correct
- The 21 pre-existing guide nav bars that had the same bug
  (all 21 of them, except the 3 that used the correct case:
  guide_mma.md, guide_simulations.md, guide_tools.md) are now
  also fixed
- Inter-guide links (e.g. [Discussions](guide_discussions.md))
  were not affected; they were always correct because both the
  link text and the actual filename are lowercase

This is a docs-only fix. No code modified.

docs/guide_api_hooks.md

@@ -1,6 +1,6 @@
 # `src/api_hooks.py` & `src/api_hook_client.py` — Hook API
 
-[Top](../README.md) | [Architecture](guide_architecture.md) | [Testing](guide_testing.md)
+[Top](../Readme.md) | [Architecture](guide_architecture.md) | [Testing](guide_testing.md)
 
 ---